JP2015078395A - HIGH-STRENGTH STEEL SHEET, HIGH-STRENGTH GALVANIZED STEEL SHEET AND HIGH-STRENGTH ALLOYED GALVANIZED STEEL SHEET HAVING MAXIMUM TENSILE STRENGTH OF 980 MPa AND EXCELLENT DELAYED FRACTURE RESISTANCE CHARACTERISTIC AND THEIR PRODUCTION METHOD - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet, a high-strength galvanized steel sheet and a high-strength alloyed galvanized steel sheet which have a maximum tensile strength(TS) of 980 MPa or higher and are applicable to impact absorption members on collision.SOLUTION: A high-strength steel sheet has a micro-structure which contains, by volume fraction, 20% or more of ferrite as the main phase and a total of 5-80% of martensite and/or bainite as the second phase and is restricted in volume fraction of retained austenite to lower than 10%. Martensite and bainite have a shape meeting equation (1), where V is the volume of martensite particles or bainite particles; and S is the surface area of martensite particles or bainite particles. The high-strength steel sheet has a maximum tensile strength of 980 MPa or higher and is excellent in delayed fracture resistance characteristics.

Description

  The present invention is a high-strength steel sheet, high-strength hot-dip galvanizing, particularly suitable for structural members, reinforcing members, and suspension members for automobiles having a maximum tensile strength (TS) of 980 MPa or more and excellent delayed fracture resistance. The present invention relates to a steel sheet and a high-strength galvannealed steel sheet. The plated steel sheet in the present invention is a galvanized steel sheet and a galvannealed steel sheet, and the plated layer contains Fe, Al, Mg, Mn, Si, Cr, Ni, Cu, etc. in addition to pure zinc. It doesn't matter.

In recent years, there has been an increasing demand for higher strength steel sheets used in automobiles and buildings, and high-strength cold-rolled steel sheets with a maximum tensile strength of 980 MPa or more are rapidly being applied to structural members of automobiles. However, it is necessary to solve the problem of delayed fracture when applying high strength steel sheets.
Delayed fracture is a phenomenon in which a member subjected to high stress, such as a PC steel wire or a bolt, suddenly breaks, and is known to be closely related to hydrogen entering from the environment.

As a result, many steel materials have been developed in consideration of delayed fracture resistance in the steel strip and plate fields. For example, in steel for steel bars and bolts, development has been conducted mainly on tempered martensite. In Non-Patent Document 1, additive elements exhibiting temper softening resistance such as Cr, Mo and V are effective in improving delayed fracture resistance. It has been reported that. This is a technique for precipitating alloy carbides and using them as hydrogen trap sites to shift the delayed fracture characteristic form from grain boundaries to intragranular fracture. However, these steels have a C content of 0.4% or more and contain a large amount of alloy elements, so the workability and weldability required for thin steel sheets are inferior, and the precipitation of alloy carbide takes several hours or more. Therefore, there is a problem in manufacturability.
In Patent Document 1, an oxide mainly composed of Ti and Mg is considered to be effective in preventing hydrogen defects. However, this is intended for thick steel plates, especially considering delayed fracture characteristics after welding with high heat input, but it is absolutely necessary to consider both high formability and delayed fracture resistance required for thin steel sheets. It has not been.

  On the other hand, (1) the thin steel plate is thin, so even if hydrogen enters, it is released in a short time, and (2) there is almost no use of a steel plate of 980 MPa or more in terms of workability. The problem with delayed fracture characteristics was small. However, since the demand for the application of high-strength steel plates is rapidly increasing, it is necessary to develop high-strength steel plates with excellent delayed fracture resistance.

  However, most of the techniques for improving the delayed fracture resistance have been developed for steel materials that are often used as products, such as bolts, strips, and thick plates, and below the yield strength or yield stress. In other words, it is not a technique that takes into consideration steel materials that require delayed fracture resistance as well as workability such as cutting and member molding (press molding) like automobile members. In particular, a stress called residual stress remains in the member after molding. Although this stress is local, it may be a high value exceeding the yield stress of the material, and it is required that hydrogen embrittlement does not occur under this high stress.

  On the other hand, regarding delayed fracture characteristics of thin steel sheets, for example, Non-Patent Document 2 reports the promotion of delayed fracture characteristics due to work-induced transformation of retained austenite. This is in consideration of the forming process of a thin steel sheet, but describes the regulation of the amount of retained austenite which does not deteriorate the delayed fracture resistance. That is, it relates to a high-strength thin steel sheet having a specific structure, and is not a fundamental measure for improving delayed fracture resistance.

  In addition, as a thin steel plate considering hydrogen trapping ability and formability, there is a steel plate for a hollow container described in Patent Document 2 that is excellent in toughness resistance. This is intended to suppress a surface defect called a trap generated after enamelling by trapping hydrogen that enters the steel sheet during production with an oxide contained in the steel sheet. Therefore, a large amount of oxide is contained inside the steel plate. However, dispersing these oxides at a high density in the steel sheet causes deterioration of formability, and therefore there is a problem in application to a steel sheet for automobiles that require high formability. In addition, these studies do not attempt to achieve both high strength and delayed fracture resistance.

  As a technique for improving the delayed fracture resistance of the molded member, a molding technique for reducing the residual stress of the molded member can be considered. However, since the residual stress remains in the molded member, there are many parts that depend on the characteristics of the steel material, and the shape of the member cannot be changed greatly. There are many problems in the method of reducing.

  In general, martensite single-phase structure strengthening is known as a technique for ensuring a high strength of 980 MPa or more. However, although the martensite structure has high strength, the uniform elongation is extremely low, the workability is poor, and even if it can be molded, there is a large residual stress in the molded member, so the delay resistance of the molded member is delayed. It had the problem that the fracture characteristics were inferior.

  As a technique for solving these problems, there is known a composite structure steel plate having a main phase of ferrite and a hard structure of martensite. That is, it is a technique of ensuring ductility with soft ferrite and ensuring strength with hard martensite. In this method, the yield stress can be significantly reduced by making the main phase soft ferrite.

  However, in order to ensure the tensile strength of 980 MPa or more of the steel sheet, it is necessary to increase the martensite volume ratio, and there is a problem that uniform elongation cannot be avoided. That is, the strength of the martensite structure is strongly dependent on the steel plate component (particularly C), and thus it is difficult to increase without changing the steel plate component. As a result, an increase in the volume ratio of martensite was indispensable for increasing the strength of the multiphase steel sheet composed of ferrite and martensite.

In addition, when performing processes such as continuous annealing and continuous hot dip galvanizing lines, inevitable heat treatment such as annealing, tempering in the overaging zone after cooling, and alloying heat treatment after immersion in the plating bath is additionally provided. By doing so, the martensitic structure is tempered and softened, so that it is difficult to ensure the strength of the steel sheet. For this reason, when manufacturing a high-strength steel sheet exceeding 980 MPa, for example, using a manufacturing facility that receives such additional heat treatment, generally, even if softening occurs due to tempering of the martensite structure. In order to ensure sufficient strength, the martensite volume fraction is further increased. As a result, further elongation deterioration is expected.
Thus, it is extremely difficult to achieve both high strength and delayed fracture resistance.

  Moreover, as a technique for increasing the strength of martensite, for example, there are techniques described in Non-Patent Documents 3 to 5. Non-Patent Document 3 discloses that the hardness (strength) of a lath-like structure (lass martensite structure), which is one of the martensite structure shapes, is the amount of solute C in martensite, crystal grain size, and carbide. It is described that it depends on precipitation strengthening and dislocation strengthening. Non-Patent Document 4 and Non-Patent Document 5 describe that the strength of martensite can be greatly increased by refining the crystal grain size, particularly the block size, which is one of the structural units constituting martensite. Yes.

  Moreover, as a prior art regarding a high strength steel plate, the technique of patent document 3-patent document 8 is mentioned, for example. Moreover, as a conventional technique regarding the hot dip galvanized steel sheet, for example, a technique described in Patent Document 9 is cited.

JP-A-11-293383 Japanese Patent Laid-Open No. 11-100638 Japanese Patent Application Laid-Open No. 11-296991 Japanese Patent Laid-Open No. 9-13147 JP 2002-363695 A JP 2003-105514 A JP 2003-213369 A JP 2003-213370 A JP 2002-97560 A

"New development of delayed fracture characteristics elucidation" (Japan Steel Association, published in January 1997) CAMP-ISIJ vol.5 No.6 pp. 1839-1842, Yamazaki et al., October 1992, published by Japan Iron and Steel Institute F. B. Pickering: Hardenability Concepts with Applications to Steel AIME (1978), p179 M. Wang: IS3-2007, p203 T. Ohmura: IS3-2007, p35

  An object of the present invention is to provide a high-strength steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength galvannealed steel sheet having a maximum tensile strength (TS) of 980 MPa or more and excellent in delayed fracture resistance. To do.

  As a result of diligent investigations, the inventors of the present invention have, as the properties of the steel sheet excellent in delayed fracture resistance, the shape of martensite or bainite contained in the microstructure of the steel plate that satisfies the following formula (1). As a result, it was found that the characteristics could be secured.

V: Volume of martensite grains
S: Surface area of martensite grains

  That is, the present invention is a high-strength steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength galvannealed steel sheet having a maximum tensile strength (TS) of 980 MPa or more and excellent in delayed fracture resistance. It is as follows.

(1) The microstructure contains 20% or more of ferrite as a main phase in volume fraction, and contains 30 to 80% or less of martensite and bainite as a second phase in total, and a retained austenite High strength excellent in delayed fracture resistance having a maximum tensile strength of 980 MPa or more, characterized in that the steel sheet restricts the volume ratio to less than 10% and martensite and bainite have a shape satisfying the following formula (1). Cold rolled steel sheet.

V: Volume of martensite grains
S: Surface area of martensite grains

(2) The steel sheet according to (1) is in mass%,
C: 0.05 to 0.40%,
Si: 0.01-3.0%,
Mn: 1.5 to less than 3.5%,
P: 0.04% or less,
S: 0.01% or less,
Al: 2.0% or less,
N: 0.01% or less,
O: A high-strength cold-rolled steel sheet excellent in delayed fracture resistance having a maximum tensile strength of 980 MPa or more, characterized by comprising 0.006% or less and the balance being iron and inevitable impurities.

(3) Furthermore, Cr: 0.05-1.0% by mass% in steel,
Mo: 0.01 to 1.0%,
Ni: 0.05 to 1.0%,
Cu: 0.05 to 1.0%,
Nb: 0.005-0.3%
Ti: 0.005-0.3%
V: 0.005 to 0.5%,
B: 0.0001 to 0.01%
Ca: 0.0005 to 0.04%,
Mg: 0.0005 to 0.04%,
REM: 0.0005 to 0.04%,
A high-strength cold-rolled steel sheet excellent in delayed fracture resistance having a maximum tensile strength of 980 MPa or more as described in (2).

(4) The maximum tensile strength of the cold-rolled steel sheet according to any one of (1) to (3) having a hot-dip galvanized layer containing less than 7% by mass of Fe and the balance of Zn, Al and inevitable impurities A hot-dip galvanized steel sheet with a delayed fracture resistance having a strength of 980 MPa or more.

(5) On the surface of the cold-rolled steel sheet according to any one of (1) to (3), Fe is contained in an amount of 7% by mass or more and 15% by mass or less, and the balance is composed of Zn, Al and inevitable impurities. A hot-dip galvanized steel sheet having a delayed fracture resistance and a maximum tensile strength of 980 MPa or more having a hot-dip galvanized layer.

(6) When casting the steel comprising the chemical component according to any one of (2) and (3), the average cooling rate on the slab surface at 1400 to 1200 ° C is 200 ° C or less. A method for producing a high-strength steel sheet having a delayed fracture resistance and a maximum tensile strength of 980 MPa or more.

(7) After carrying out the casting of the steel ingot or slab composed of the chemical component according to either (2) or (3) under the conditions of (6), directly or once cooled, and then heated to 1100 ° C or higher, When hot rolling is completed at the Ar3 transformation point or higher, winding is performed in a temperature range of 700 ° C. or lower, pickling is performed, cold rolling is performed at a reduction rate of 30 to 80%, and then a continuous annealing line is passed through. , Annealing at 750 ° C. or more and 900 ° C. or less, then cooling to 500 to 750 ° C. at 0.5 to 200 ° C./second, cooling to 100 to 450 ° C. at a cooling rate of 1 ° C./second or more, Excellent delayed fracture resistance with a maximum tensile strength of 980 MPa or more, characterized by performing reheating, holding, or cooling, holding for 10 to 1000 seconds at 150 to 500 ° C., and then cooling to room temperature A method for manufacturing cold rolled steel sheets.

(8) After carrying out the casting of the steel ingot or slab composed of the chemical component according to any one of (2) and (3) under the conditions of (6), directly or once cooled and then heated to 1100 ° C or higher, Hot rolling is completed at the Ar3 transformation point or higher, winding in a temperature range of 700 ° C. or lower, pickling, cold rolling at a rolling reduction of 30 to 80%, then passing through a continuous hot dip galvanizing line In this case, annealing is performed at 750 ° C. or more and 900 ° C. or less, and then cooled to 500 to 750 ° C. at 0.5 to 200 ° C./second, at a cooling rate of 1 ° C./second or more (zinc plating bath temperature− 40) A method for producing a hot-dip galvanized steel sheet having a delayed fracture resistance having a maximum tensile strength of 980 MPa or more, characterized by cooling to room temperature after cooling to 40 ° C. to (zinc plating bath temperature + 50) ° C.

(9) After carrying out the casting of the steel ingot or slab composed of the chemical component according to either (2) or (3) under the conditions of (6), directly or once cooled, and then heated to 1100 ° C or higher, Hot rolling is completed at the Ar3 transformation point or higher, winding in a temperature range of 700 ° C. or lower, pickling, cold rolling at a rolling reduction of 30 to 80%, then passing through a continuous hot dip galvanizing line In this case, annealing is performed at 750 ° C. or more and 900 ° C. or less, and then cooled to 500 to 750 ° C. at 0.5 to 200 ° C./second, and at a cooling rate of 1 ° C./second or more to between 500 ° C. and room temperature. After cooling, it is heated to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C., or after cooling, galvanized and then cooled to room temperature, and has a maximum tensile strength of 980 MPa or more. Excellent delayed fracture resistance Manufacturing method of hot-dip galvanized steel sheet.

(10) After the heat treatment and plating are performed by the method described in (2) or (3), the alloying treatment is performed in the range of 460 to 600 ° C., and then cooled to room temperature. A method for producing an inter-alloy hot-dip galvanized steel sheet having a delayed fracture resistance with a strength of 980 MPa or more.

(11) After heat treatment or plating by the method according to any one of (7) to (10), after cooling to 100 ° C. or lower, reheating to a temperature range of 150 to 600 ° C. A method for producing a high-strength steel sheet having a delayed tensile fracture resistance having a maximum tensile strength of 980 MPa or more, characterized by cooling to room temperature after performing a heat treatment for 2 seconds.

  The present invention provides a high-strength steel sheet excellent in delayed fracture resistance having a tensile maximum strength of 980 MPa or more with a tensile strength suitable for shock absorbing members, structural members, reinforcing members, and suspension members for automobiles at low cost. it can.

  As a result of intensive studies to solve the above problems, the inventors of the present invention have excellent bendability by controlling the shape of martensite and bainite contained in the microstructure of the steel sheet within the range of the following formula (1). I found that I can secure.

V: Volume of martensite grains or bainite grains
S: Surface area of martensite grains or bainite grains

The contents of the present invention will be described in detail below.
First, when the relationship between the shape of martensite or bainite grains contained in the steel sheet and the delayed fracture resistance is investigated in detail, the shape of martensite or bainite grains is in the range of formula (1). It was found that the maximum tensile strength of 980 MPa or more and the delayed fracture resistance can be compatible. Although the detailed mechanism is unknown, there is a strain introduced at the time of transformation at the ferrite / martensite interface or the ferrite / bainite interface, so it is estimated that these strain fields acted as hydrogen trap sites. The This becomes remarkable when the surface area and volume ratio of martensite or bainite satisfy the formula (1). The smaller the left side of the formula (1), the larger the surface area of martensite or bainite grains, and the dissolved hydrogen in the steel material will not concentrate in the local region and cause hydrogen cracking. It was considered. That is, since the ratio of the unit volume is increased, it is considered that the function as a trap site is improved and the delayed fracture resistance is improved.

  The formula (1) is a formula obtained by the present inventors as a result of careful investigation of the relationship between the microstructure of the steel sheet and delayed fracture.

  The reason why the value of the expression (1) is set to 0.008 or less is to make martensite or bainite into a complex shape and improve delayed fracture characteristics. When considering both the maximum tensile strength of 980 MPa or more and the delayed fracture resistance, the numerical value of the formula (1) needs to be 0.008 or less. Therefore, the upper limit is set to 0.008. Moreover, it is desirable to satisfy the formula (1) also from the viewpoint of dispersion of the concentrated concentration of deformation that becomes the ductile fracture starting point. On the other hand, if it exceeds 0.008, the delayed fracture resistance improvement effect is small and the delayed fracture resistance is inferior.

  The three-dimensional geometric shape of martensite or bainite can be measured by any method as long as the three-dimensional shape of martensite or bainite can be measured. For example, the microstructure of a steel sheet may be observed three-dimensionally by polishing in a scanning electron microscope (SEM) using a field emission ion beam (FIB) or by a serial sectioning method using mechanical polishing and etching. .

  Or you may observe the three-dimensional shape of a martensite and a bainite, without destroying a steel plate by using a radiated light. In this study, a serial sectioning method that combines mechanical polishing, etching, and optical microscopy was performed for simplicity and evaluation of large-volume structures. In the observation, the steel sheet was cut out in parallel with the rolling direction, polished and etched, and the structure was observed with an optical microscope. Then, polishing, etching and structural observation with an optical microscope were repeated every 0.5 μm. At this time, the tissue observation position was set to 1/4 of the thickness in the thickness direction, and the obtained images were superimposed to reconstruct the three-dimensional structure.

  As the observation area, the three-dimensional morphology of martensite grains and bainite grains in the region of 100 μm × 100 μm × 100 μm was evaluated. Since this steel sheet is 980 MPa or more, the average grain size of martensite and ferrite in a two-dimensional cross section parallel to the rolling direction is as small as 5 μm or less, and there are many martensite grains and ferrite in this measurement volume. Grain was included. From this, it was considered that the representative volume is sufficient to evaluate the three-dimensional form of the structure of the steel sheet.

Next, the reason for limiting the volume ratio of each tissue in the present invention will be described.
The volume ratio of ferrite as the main phase is set to 20% or more. The reason why the ferrite volume fraction is set to 20% or more is to ensure good elongation. If the ferrite volume fraction is less than 20%, the work hardening becomes too low, the moldability becomes low, and the moldability is poor. On the other hand, when the ferrite volume fraction exceeds 95%, it is difficult to ensure the strength of 980 MPa or more. For this reason, the ferrite volume fraction needs to be 20 to 70%.

  The reason why the total volume ratio of martensite or bainite, which is a strengthened structure, is 30 to 80% is to ensure a maximum tensile strength of 980 MPa or more. When the volume ratio is less than 30%, the maximum tensile strength is less than 980 MPa, and the occurrence of delayed fracture is difficult to occur. On the other hand, if the volume ratio exceeds 80%, the martensite and bainite volume ratios are too large, and the moldability is poor. For this reason, the total of the martensite and bainite volume fractions needs to be 80% or less.

  The martensite is a tempered martensite containing iron-based carbide (such as cementite or ε carbide) in its interior, or fresh martensite containing no carbide, and the three-dimensional shape that is the condition of the present invention If the volume ratio is satisfied, the effects of the present invention can be obtained.

  The bainite structure is either an upper bainite having cementite between lath-shaped ferrites constituting the bainite structure, a lower bainite having iron-based carbide in the lath, or a mixed structure of lath-shaped ferrite and austenite, The effect of the present invention can be obtained as long as the three-dimensional shape and volume ratio which are the conditions of the present invention are satisfied.

  The residual austenite volume fraction should be limited to less than 10%. Residual austenite transforms into martensite during press molding, resulting in excellent work hardening and high uniform elongation. However, martensite transformed from retained austenite is extremely hard and easily becomes a starting point of voids and cracks due to concentration of deformation. As a result, the delayed fracture resistance is deteriorated. Therefore, it is necessary to limit the residual austenite volume fraction to less than 10%.

  As a structure other than ferrite, martensite, or retained austenite, iron-based carbides such as pearlite and cementite may be contained.

  Incidentally, the identification of each phase of the above microstructure, ferrite, martensite, bainite, austenite, pearlite and the remaining structure, observation of the existing position and measurement of the area ratio are disclosed in Nital reagent and JP-A-59-219473. It is possible to corrode the steel plate rolling direction cross section or the rolling direction perpendicular direction cross section with the above-mentioned reagent, and to quantify by 1000 times optical microscope observation and 1000 to 100000 times scanning and transmission electron microscopes.

  Further, in the three-dimensional structure observation of martensite grains and bainite grains by serial sectioning, the volume ratio of each structure included in the representative volume of 100 μm × 100 μm × 100 μm is set to the volume ratio of each structure included in the steel sheet of the present invention. It is also good. In the present invention, the volume ratio of each structure included in the representative volume observed by serial sectioning is defined as the volume ratio of each structure included in the steel sheet of the present invention.

  The particle size is not particularly limited as long as the shape of the present invention is satisfied. In particular, in the observation of the structure in a two-dimensional cross section, only a part of the martensite grains and bainite grains is observed, and in many cases does not coincide with the actual particle diameter. In particular, the martensite grains and bainite grains contained in the steel sheet of the present invention are controlled in a three-dimensionally complex shape by controlling the manufacturing conditions.

  As a result, a two-dimensional cross section often has a complicated shape in three dimensions, even if it is fine and equiaxed. As a result, the major axis of the three-dimensional grain is large. From this, the index showing the shape shown by Formula (1) is more important than the particle size values of martensite and bainite.

  The steel sheet needs to have a maximum tensile strength of 980 MPa or more. This is because a steel sheet having a maximum tensile strength of less than 980 MPa has sufficient delayed fracture resistance, and there is very little concern about the occurrence of delayed fracture. Even if it is less than 980 MPa, since the further characteristic improvement is caused by setting it as the shape of the martensite grain or bainite grain of the present invention, it is desirable for improving the characteristic.

Next, the plating layer will be described.
Since corrosion resistance increases by having a plating layer in a steel plate, you may plate.
When spot weldability or paintability is desired, these characteristics can be enhanced by alloying treatment. Specifically, by immersing in a Zn plating bath and then performing an alloying treatment, Fe is taken into the plating layer, and a high-strength hot-dip galvanized steel sheet excellent in paintability and spot weldability can be obtained. . If the amount of Fe after alloying is less than 7% by mass, spot weldability is insufficient. On the other hand, if the amount of Fe exceeds 15% by mass, the adhesion of the plating layer itself is impaired, and during processing, the plating layer breaks and falls off and adheres to the mold, which causes defects during molding. Therefore, the range of the amount of Fe in the plating layer when the alloying treatment is performed is 7 to 15% by mass.

  Further, when the alloying treatment is not performed, even if the Fe amount in the plating layer is less than 7% by mass or less, the corrosion resistance and delayed fracture resistance, which are the effects excluding spot welding obtained by alloying, are good.

The plating adhesion amount is not particularly limited, but is preferably 5 g / m ² or more in terms of single-sided adhesion from the viewpoint of corrosion resistance. For the purpose of improving the paintability and weldability on the hot-dip Zn plated steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. However, the present invention does not depart from the present invention.

  Further, in order to further improve the plating adhesion, the present invention does not depart from the present invention even if the steel plate is plated with Ni, Cu, Co, or Fe alone or before the annealing.

Furthermore, regarding annealing before plating, “after degreasing and pickling, heating in a non-oxidizing atmosphere, annealing in a reducing atmosphere containing H ₂ and N ₂ , cooling to near the plating bath temperature, and immersing in the plating bath "Zenzimer method", "All-reduction furnace method of adjusting the atmosphere during annealing, first oxidizing the surface of the steel sheet, and then reducing it before cleaning by plating before cleaning," or There is a flux method such as “after degreasing and pickling the steel plate, and then fluxing it with ammonium chloride and soaking it in a plating bath”, etc. Can demonstrate.

  Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc. Moreover, when alloying a plating layer, it carries out at 460 degreeC or more. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. The upper limit is not particularly limited, but if it exceeds 600 ° C., carbides are formed and the volume fraction of hard structure (martensite, bainite, retained austenite) is reduced, and it becomes difficult to ensure excellent ductility. is there.

  Moreover, when manufacturing an galvannealed steel sheet, in order to control the characteristic of a plating layer, it is desirable to control the effective Al density | concentration in a plating bath in the range of 0.05-0.500 mass%. Here, the effective Al concentration in the plating bath is a value obtained by subtracting the Fe concentration in the bath from the Al concentration in the bath.

  The reason for limiting the effective Al concentration to 0.05 to 0.500 mass% is that dross generation is remarkable and a good appearance cannot be obtained when the effective Al concentration is lower than 0.05 mass%. On the other hand, when the effective Al concentration is higher than 0.500 mass%, alloying is slow and productivity is inferior. Therefore, the upper limit of the effective Al concentration in the bath is desirably 0.500% by mass.

  In order to measure the content of Fe and Al in the plating layer, a method of dissolving the plating layer with an acid and chemically analyzing the solution may be used. For example, an alloyed hot-dip galvanized steel sheet cut to 30 mm × 40 mm is dissolved with a 5% HCl aqueous solution to which an inhibitor is added while only the plating layer is dissolved while suppressing elution of the steel sheet base material. A method of quantifying the contents of Fe and Al from the obtained signal intensity and a calibration curve created from a solution having a known concentration may be used. In addition, in consideration of measurement variations among the samples, an average value obtained by measuring at least three samples cut out from the same alloyed hot-dip galvanized steel sheet may be employed.

Next, the reasons for limiting the components will be described. In addition,% means the mass%.
C: C is an element that can increase the strength of the steel sheet. However, if it is less than 0.05%, it becomes difficult to achieve both a tensile strength of 980 MPa or more and workability. On the other hand, if it exceeds 0.40%, it becomes difficult to ensure spot weldability. For this reason, the range was limited to 0.05 to 0.40% or less.

  Si: Si is a strengthening element and is effective in increasing the strength of the steel sheet. In addition, it contributes to higher strength and improved bendability through the suppression of cementite precipitation and coarsening. However, if it is less than 0.01%, the effect of increasing the strength is small, and if it exceeds 3.0%, the workability deteriorates. Therefore, the Si content is limited to a range of 0.01 to 3.0%.

  Mn: Mn is a strengthening element and is effective in increasing the strength of the steel sheet. However, if it is less than 1.5%, it is difficult to obtain a tensile strength of 980 MPa or more. On the contrary, if the amount is too large, co-segregation with P and S is promoted, and the workability is significantly deteriorated. A more preferable range is 1.8 to 3.2%.

  O: O forms an oxide and degrades elongation, bendability and hole expandability, so the addition amount needs to be suppressed. In particular, oxides often exist as inclusions, and when they are present on the punched end surface or cut surface, they form notched scratches and coarse dimples on the end surface, so when expanding holes or during strong processing, It causes stress concentration and becomes the starting point of crack formation, resulting in a significant deterioration of hole expansibility or bendability. This is because when O exceeds 0.006%, this tendency becomes remarkable. Therefore, the upper limit of the O content is set to 0.006% or less. If the content is less than 0.0001%, an excessively high cost is caused and this is not economically preferable, so this is a practical lower limit.

  P: P tends to segregate in the central part of the plate thickness of the steel sheet and embrittles the weld. When the content exceeds 0.04%, the weld becomes brittle, so the appropriate range is limited to 0.04% or less. Although the lower limit value of P is not particularly defined, it is preferable to set this value as the lower limit value because it is economically disadvantageous to set it to less than 0.0001%.

  S: S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit is set to 0.01% or less. Although the lower limit of S is not particularly defined, it is preferable to set this value as the lower limit because it is economically disadvantageous to make it less than 0.0001%. Further, since S is combined with Mn to form coarse MnS, the bendability and hole expansibility are deteriorated, so it is necessary to reduce it as much as possible.

  Al: Al may be added because it promotes ferrite formation and improves ductility. It can also be used as a deoxidizer. However, excessive addition increases the number of Al-based coarse inclusions, causing deterioration of hole expansibility and surface scratches. From this, the upper limit of Al addition was set to 2.0%. The lower limit is not particularly limited, but it is difficult to set it to 0.0005% or less, so this is a substantial lower limit.

  N: N forms coarse nitrides and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. This is because when N exceeds 0.01%, this tendency becomes remarkable. Therefore, the range of N content is set to 0.01% or less. In addition, it is better to use less because it causes blowholes during welding. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased, and this is a substantial lower limit.

  Mo: Mo is a strengthening element and is important for improving hardenability. However, if it is less than 0.01%, these effects cannot be obtained, so the lower limit was set to 0.01%. On the other hand, if the content exceeds 1%, the manufacturability at the time of production and hot rolling is adversely affected, so the upper limit was made 1%.

  Cr: Cr is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. On the other hand, if the content exceeds 1%, the manufacturability at the time of production and hot rolling is adversely affected, so the upper limit was made 1%.

  Ni: Ni is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. On the other hand, if the content exceeds 1%, the manufacturability at the time of production and hot rolling is adversely affected, so the upper limit was made 1%. In addition, it may be added because it improves wettability and promotes the alloying reaction.

  Cu: Cu is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. On the other hand, if the content exceeds 1%, the manufacturability at the time of production and hot rolling is adversely affected, so the upper limit was made 1%. In addition, it may be added because it improves wettability and promotes the alloying reaction.

  B is effective for strengthening grain boundaries and strengthening steel by addition of 0.0001% or more, but when the addition amount exceeds 0.01%, the effect is not only saturated but also during hot rolling. The upper limit is set to 0.01% because manufacturing is reduced.

  Ti: Ti is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the added amount is less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. If the content exceeds 0.3%, carbonitride precipitation increases and the formability deteriorates, so the upper limit was made 0.3%.

  Nb: Nb is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the added amount is less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. If the content exceeds 0.3%, carbonitride precipitation increases and the formability deteriorates, so the upper limit was made 0.3%.

  V: V is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the added amount is less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. If the content exceeds 0.5%, carbonitride precipitation increases and the formability deteriorates, so the upper limit was made 0.5%.

  One or more selected from Ca, Mg, and REM can be added in a total amount of 0.0005 to 0.04%. Ca, Mg, and REM are elements used for deoxidation, and it is preferable to contain one or two or more in total of 0.0005% or more. REM is Rare Earth Metal. However, when the content exceeds 0.04% in total, it causes deterioration of molding processability. Therefore, the total content is set to 0.0005 to 0.04%. In the present invention, REM is often added by misch metal and may contain lanthanoid series elements in combination with La and Ce. Even if these lanthanoid series elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited. However, the effects of the present invention are exhibited even when metal La or Ce is added.

  The manufacturing method preceding casting is not particularly limited. That is, various secondary smelting may be performed following the smelting by a blast furnace or an electric furnace. Next, the average cooling rate of the slab surface during casting needs to be 200 ° C./second or less. The average cooling rate of the slab surface during casting is one of the most important conditions in the steel sheet of the present invention. That is, in order to control martensite and bainite to the shape defined in (1), it is necessary to control the Mn microsegregation during casting and to control the shape of martensite and bainite.

  However, since a high strength steel plate of 780 MPa or more contains a large amount of alloy elements, the average cooling rate of the slab surface during casting is set to 200 ° C./second or more for the purpose of suppressing slab cracking during continuous casting called breakout. It was necessary to quickly solidify. However, although production at a high cooling rate reduces the risk of breakout during continuous casting, it reduces the primary dendrite texture formed during solidification. As a result, martensite and bainite have a simple form, do not satisfy the formula (1), and bendability deteriorates. For this reason, the cooling rate on the slab surface at 1400 to 1200 ° C. during casting needs to be 200 ° C./second or less.

  The cast slab may be once cooled to a low temperature and then heated again and then hot rolled, or the cast slab may be continuously hot rolled. Scrap may be used as a raw material.

  Further, the cooling after rolling is not particularly defined, and the effect of the present invention can be obtained even if a cooling pattern for controlling the structure suitable for each purpose is taken. The winding temperature needs to be 700 ° C. or lower. When the temperature exceeds 700 ° C., coarse ferrite and pearlite structures exist in the hot-rolled structure, so that the structure non-uniformity after annealing increases and the material anisotropy of the final product increases. The structure after annealing is refined to improve the strength ductility balance.

  In addition, winding at a temperature exceeding 700 ° C. is not preferable because the thickness of the oxide formed on the steel sheet surface is excessively increased, resulting in poor pickling properties. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, since it is technically difficult to wind up at a temperature of room temperature or lower, this is the actual lower limit. Note that rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once.

  The hot-rolled steel sheet thus manufactured is pickled. Pickling is important for improving plating properties because it can remove oxides on the surface of the steel sheet. Moreover, pickling may be performed once, or pickling may be performed in a plurality of times.

  The pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 30 to 80% and passed through a continuous hot dip galvanizing line. If the rolling reduction is less than 30%, it is difficult to keep the shape flat. Moreover, since the ductility of the final product becomes poor, this is the lower limit. On the other hand, the cold rolling exceeding 80% makes the cold rolling difficult because the cold rolling load becomes too large. 40 to 70% is a more preferable range. The effect of the present invention is exhibited without particularly specifying the number of rolling passes and the rolling reduction for each pass.

  The effect of the present invention is exhibited without any particular limitation on the heating rate when passing through the continuous annealing line or the plating line. A heating rate of less than 0.5 ° C./second is not preferable because productivity is greatly impaired, and this is the lower limit. On the other hand, setting the heating rate to more than 100 ° C./second invites excessive capital investment and is not economically preferable, so this is a practical upper limit.

  The maximum heating temperature is in the range of 750 to 900 ° C. When the maximum heating temperature is less than 750 ° C., it takes too much time for the carbide formed at the time of hot rolling to re-dissolve, and it is difficult to ensure a strength of 980 MPa or more because the carbide or a part thereof remains. . Therefore, 750 ° C. is the lower limit of the maximum heating temperature. On the other hand, excessively high temperature heating not only is economically undesirable because it leads to an increase in cost, but also induces troubles such as deterioration of the plate shape at the time of hot plate passing and reduction in the life of the roll. Therefore, the upper limit of the maximum heating temperature is set to 900 ° C. The heat treatment time in this temperature range is not particularly limited, but a heat treatment of 10 seconds or more is desirable for dissolving the carbide. On the other hand, if the heat treatment time exceeds 600 seconds, the cost increases, which is not economically preferable. Regarding the heat treatment, the isothermal holding may be performed at the maximum heating temperature, or even if cooling is started immediately after the gradient heating is performed and the maximum heating temperature is reached, the effect of the present invention is exhibited.

  After completion of the annealing, it is cooled to 500 to 750 ° C. The average cooling rate from the highest heating temperature to 500 to 750 ° C. is desirably 0.5 to 200 ° C./second. When the cooling rate is less than 0.5 ° C./second, austenite is transformed into a pearlite structure in the cooling process, or a large amount of ferrite is formed and the maximum tensile strength is less than 980 MPa. Or it becomes difficult to make the bainite volume ratio 30% or more, so the lower limit was made 0.5 ° C./second or more. Even if the cooling rate is increased, there is no problem in terms of the material. However, excessively increasing the cooling rate leads to an increase in manufacturing cost, so the upper limit is preferably set to 200 ° C./second. The cooling method may be roll cooling, air cooling, water cooling, or any combination of these methods.

Then, martensite and bainite are formed by cooling from 500 ° C. to room temperature. When the cooling stop temperature is higher than 500 ° C., austenite is transformed into pearlite, so the total volume ratio of martensite and bainite cannot be made 30% or more, and it is difficult to secure a strength of 980 MPa or more. For this reason, the upper limit of the cooling stop temperature is set to 500 ° C. Cooling to a temperature range below room temperature is not preferable because not only the effect is saturated but also excessive capital investment is required. Alternatively, when tempering is performed in order to improve martensite characteristics, it must be reheated to a temperature range of 250 to 500 ° C., which is inferior in economic efficiency. Therefore, it is desirable that the cooling stop temperature is in a temperature range of 500 to 150 ° C.
Subsequently, tempering for improving martensite characteristics is performed by maintaining the temperature between 500 and 250 ° C. for 10 to 1000 seconds. It is necessary to carry out this heat treatment because the hole expandability and bendability are improved by tempering martensite and the delayed fracture resistance is further improved. The upper limit of the holding temperature is set to 500 ° C. because tempering at a temperature higher than this temperature causes a significant decrease in martensite strength and it is difficult to ensure a strength of 980 MPa or higher. On the other hand, holding at a temperature of less than 250 ° C. takes a long time to improve the martensite characteristics, resulting in excessive facilities and inferior productivity. Therefore, the holding temperature needs to be 500 to 250 ° C. The lower limit is set to 10 seconds. If the holding time is less than 10 seconds, the martensite characteristics are not sufficiently improved by tempering, and excellent moldability cannot be obtained. On the other hand, holding for more than 1000 seconds is not preferable because productivity decreases. Note that holding does not only mean isothermal holding, but also includes slow cooling and heating in this temperature range.
Moreover, when immersing in a plating bath or alloying treatment of plating after tempering, these treatments can be utilized for tempering martensite and promoting bainite transformation. The plating bath immersion plate temperature is preferably in a temperature range from a temperature 40 ° C. lower than the hot dip galvanizing bath temperature to a temperature 50 ° C. higher than the hot dip galvanizing bath temperature.

  If the bath immersion plate temperature is lower than (hot dip galvanizing bath temperature −40) ° C., the heat removal at the time of immersion in the plating bath is large, and a part of the molten zinc may solidify to deteriorate the plating appearance. The lower limit is (hot dip galvanizing bath temperature −40) ° C. However, even if the plate temperature before immersion is lower than (hot dip galvanizing bath temperature −40) ° C., reheating is performed before immersion in the plating bath, and the plate temperature is set to (hot dip galvanizing bath temperature −40) ° C. or higher. It may be immersed in.

  On the other hand, if the plating bath immersion temperature exceeds (hot dip galvanizing bath temperature +50) ° C., operational problems accompanying the rise of the plating bath temperature are induced. Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc.

  Moreover, when alloying a plating layer, it carries out at 460 degreeC or more. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. When the temperature exceeds 600 ° C., coarse carbides are formed, the moldability deteriorates, and the strength is significantly reduced. Therefore, it is difficult to ensure the maximum tensile strength of 980 MPa or more and excellent ductility, so this is the upper limit. .

  After the annealing or after the plating treatment, it may be once cooled to 100 ° C. or lower, reheated and heat-treated at 150 to 500 ° C. The heat treatment at 150 to 500 ° C. can improve the formability by tempering martensite. When the temperature is lower than 150 ° C., the effect is small. On the other hand, tempering at over 500 ° C. is not preferable because martensite is excessively tempered and it is difficult to secure a strength of 980 MPa or more.

The heat treatment time needs to be 1 to 1000000 seconds. If it is less than 1 second, it is difficult to obtain an effect, so that it is set to 1 second or more. The heat treatment exceeding 1000000 seconds not only saturates the effect but also is inferior in economic efficiency, so the upper limit was set to 1000000 seconds. In addition, when performing heat treatment for a short time of 1000 seconds or less, the heat treatment temperature is desirably 250 ° C. or higher. On the other hand, if the heat treatment is longer than 1000 seconds, the same effect can be obtained even at a low temperature of 150 to 250 ° C.
As long as the above heat treatment is possible, any heat treatment may be used. For example, heat treatment by placing a coil once wound in a box furnace, or heat treatment using an in-line heating furnace or induction heater may be performed.

  The reduction ratio of the skin pass rolling after the heat treatment is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. Since productivity will fall remarkably when it exceeds 1.5%, this is made an upper limit. The skin pass may be performed inline or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.

  Moreover, the material of the high strength cold-rolled steel sheet and the high ductility hot-dip galvanized steel sheet having a tensile maximum strength of 980 MPa or more and excellent delayed fracture characteristics of the present invention is a refining, steelmaking, casting, In principle, it is manufactured through a rolling and cold rolling process, but even if it is manufactured by omitting a part or all of it, the effects of the present invention can be obtained as long as the conditions according to the present invention are satisfied. it can.

(Example)
Next, the present invention will be described in detail with reference to examples.
A slab having the components shown in Table 1 is heated to 1240 ° C., hot-rolled under the hot rolling conditions shown in Table 2, and water-cooled in a water-cooled zone, and then wound up at the temperatures shown in Tables 2 and 3. Processed. The thickness of the hot rolled plate was in the range of 2.5 to 3.0 mm. After pickling the hot-rolled sheet, the sheet was cold-rolled at a predetermined cold rolling rate so that the sheet thickness after cold rolling was 1.2 mm to obtain a cold-rolled sheet.

  Steel types F to S in Table 1 are steel types of components defined in the present invention, and a to d are comparative examples in which the contents of C, Si, and Mn are out of range.

Thereafter, these cold-rolled sheets were subjected to heat treatment and hot dip galvanizing treatment in the continuous annealing equipment and the continuous alloying hot dip galvanizing equipment under the conditions shown in Table 2. After cooling from the annealing temperature to 500 to 750 ° C. at the cooling rate shown in Table 2, after holding in the temperature range of 500 to 250 ° C. for 5 to 300 seconds, it is immersed in a galvanizing bath controlled to a predetermined condition. And then cooled to room temperature. The effective Al concentration in the plating bath in the plating bath was in the range of 0.09 to 0.17% by mass. Some steel sheets were immersed in a galvanizing bath, then alloyed under various conditions, and cooled to room temperature. The weight per unit area was about 35 g / m ^{2 on} both sides. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.4%.

  In the tensile test, a JIS No. 5 test piece was taken in a direction perpendicular to and parallel to the rolling direction, and the tensile properties were evaluated.

  The alloying reactions were evaluated as follows.

Fe% in the plating layer was evaluated as follows.
First, with respect to a galvanized steel sheet cut to 30 mm × 40 mm, a 5% HCl aqueous solution to which an inhibitor is added dissolves only the plating layer while suppressing elution of the steel sheet base material, and the dissolved solution is analyzed by ICP emission spectrometry. The Fe% was evaluated. In consideration of measurement variations among samples, three samples were cut out from the same galvanized steel sheet, and the average of the measured values was defined as Fe%.

Next, the delayed fracture characteristics were examined by the following method.
First, the obtained steel plate was shear-cut to make a test piece of 1.2 mm × 30 mm × 100 mm whose longitudinal direction is the direction perpendicular to the rolling direction, and the end face was mechanically ground. Thereafter, the test piece was bent by a push bending method to produce a bending test piece having a radius of 5R. The opening amount of the bending test piece after stress unloading was 40 mm. Thereafter, a strain gauge was attached to the surface and tightened with a bolt to elastically deform the bending test piece, and the amount of strain at that time was read to calculate the load stress. Thereafter, the bending test piece was immersed in an aqueous solution of ammonium thiocyanate and subjected to an electrolytic charge under the condition of a current density of 1.0 mA / cm ² , thereby conducting a delayed fracture acceleration test for allowing hydrogen to penetrate into the steel sheet.
Then, even when the electrolytic charging time was 100 hours, a steel plate having no cracks was evaluated as a steel plate having good (O) delayed fracture resistance, and a steel plate having cracks was evaluated as defective (x).
The results are shown in Table 3 together with the value of equation (1).
The steel plate of the present invention has suitable properties in delayed fracture resistance.

  The present invention provides inexpensively a high-strength hot-dip galvanized steel sheet excellent in delayed fracture resistance having a maximum tensile strength of 980 MPa or more, which is suitable for structural members, reinforcing members, and suspension members for automobiles. It can be expected to contribute greatly to the weight reduction of automobiles, and the industrial effect is extremely high.

Claims (11)

The microstructure contains 20% or more of ferrite as a main phase in volume fraction, and contains 30 to 80% or less in total of one or more of martensite and bainite as the second phase, and the residual austenite volume fraction A high-strength cold-rolled steel sheet excellent in delayed fracture resistance having a maximum tensile strength of 980 MPa or more, characterized in that the steel sheet is limited to less than 10%, and martensite and bainite have a shape satisfying the following formula (1) .

V: Volume of martensite grains or bainite grains
S: Surface area of martensite grains or bainite grains The steel sheet according to claim 1 is mass%,
C: 0.05 to 0.40%,
Si: 0.01-3.0%,
Mn: 1.5 to less than 3.5%,
P: 0.04% or less,
S: 0.01% or less,
Al: 2.0% or less,
N: 0.01% or less,
O: 0.006 or less,
A high-strength cold-rolled steel sheet excellent in delayed fracture resistance having a maximum tensile strength of 980 MPa or more, characterized by comprising iron and the balance iron and inevitable impurities. Furthermore, Cr: 0.05-1.0% by mass% in steel,
Mo: 0.01 to 1.0%,
Ni: 0.05 to 1.0%,
Cu: 0.05 to 1.0%,
Nb: 0.005-0.3%
Ti: 0.005 to 0.3%,
V: 0.005 to 0.5%,
B: 0.0001 to 0.01%
Ca: 0.0005 to 0.04%,
Mg: 0.0005 to 0.04%,
REM: 0.0005 to 0.04%,
The high-strength cold-rolled steel sheet excellent in delayed fracture resistance having a maximum tensile strength of 980 MPa or more according to claim 2, comprising one or more of the following.   A tensile maximum strength of 980 MPa or more having a hot-dip galvanized layer containing less than 7 mass% of Fe on the surface of the cold-rolled steel sheet according to any one of claims 1 to 3 and comprising the balance of Zn, Al and inevitable impurities. High strength hot-dip galvanized steel sheet with excellent delayed fracture resistance.   The surface of the cold-rolled steel sheet according to any one of claims 1 to 3 has an alloyed hot-dip galvanized layer containing 7 mass% or more and 15% or less of Fe, with the balance being Zn, Al, and unavoidable impurities. A hot dip galvanized steel sheet having a delayed fracture resistance and a maximum tensile strength of 980 MPa or more.   The maximum tensile strength of 980 MPa or more, characterized in that the average cooling rate on the slab surface at 1400 to 1200 ° C. is 200 ° C. or lower when casting the steel comprising the chemical component according to claim 2 or 3. A method for producing a high-strength cold-rolled steel sheet having excellent delayed fracture resistance.   After the steel ingot or slab comprising the chemical component according to claim 2 or 3 is cast under the conditions of claim 6, the steel ingot or slab is directly or once cooled and then heated to 1100 ° C. or higher and above the Ar3 transformation point. Completing hot rolling, winding in a temperature range of 700 ° C. or lower, pickling, cold rolling with a rolling reduction of 30 to 80%, and then passing through a continuous annealing line at 750 ° C. or higher and Annealing at 900 ° C. or less, then cooling to 500 to 750 ° C. at 0.5 to 200 ° C./second, cooling to room temperature to 500 ° C. at a cooling rate of 1 ° C./second or more, reheating, holding, Alternatively, it is cooled, held at 250 to 500 ° C. for 10 to 1000 seconds, and then cooled to room temperature, and has a maximum tensile strength of 980 MPa or more and high strength molten zinc excellent in delayed fracture resistance Manufacturing method of plated steel sheet   After the steel ingot or slab comprising the chemical component according to claim 2 or 3 is cast under the conditions of claim 6, the steel ingot or slab is directly or once cooled and then heated to 1100 ° C. or higher and above the Ar3 transformation point. Completing hot rolling, winding in a temperature range of 700 ° C. or lower, pickling, cold rolling with a reduction rate of 30 to 80%, and then passing through a continuous hot dip galvanizing line at 750 ° C. And annealing at 900 ° C. or less, and then cooling to 500 to 750 ° C. at 0.5 to 200 ° C./second, at a cooling rate of 1 ° C./second or more (zinc plating bath temperature −40) ° C. to ( A method for producing a high-strength hot-dip galvanized steel sheet having a delayed fracture resistance and a maximum tensile strength of 980 MPa or more, characterized by cooling to a galvanizing bath temperature +50) ° C. and then cooling to room temperature.   A steel ingot or slab comprising the chemical component according to any one of claims 2 and 3, after casting under the conditions of claim 6, directly or once cooled, and then heated to 1100 ° C or higher and heated above the Ar3 transformation point. Complete the hot rolling, wind up in the temperature range of 700 ° C or lower, pickle, then cold-roll with a rolling reduction of 30-80%, then pass through the continuous hot dip galvanizing line at 750 ° C or higher And after annealing at 900 ° C. or less, after cooling to 500-750 ° C. at 0.5-200 ° C./second and at a cooling rate of 1 ° C./second or more, cooling between 500 ° C. and room temperature, (zinc Plating bath temperature -40) ° C to (Zinc plating bath temperature +50) ° C Heated or cooled, after galvanizing, cooled to room temperature, with delayed fracture resistance having a maximum tensile strength of 980 MPa or more Excellent high strength solution Method of manufacturing a galvanized steel sheet.   The maximum tensile strength of 980 MPa, wherein after the heat treatment and plating are performed by the method according to claim 8 or 9, the alloying treatment is performed in a range of 460 to 600 ° C. and then cooled to room temperature. A method for producing a high-strength galvannealed steel sheet having excellent delayed fracture resistance as described above.   After heat-treating or plating by the method of any one of Claims 7 thru | or 10, after cooling to 100 degrees C or less, it reheats to the temperature range of 150-600 degreeC, and heat-treats for 1 to 1000000 seconds. And then cooling to room temperature, and a method for producing a high-strength galvannealed steel sheet having excellent delayed fracture resistance having a maximum tensile strength of 980 MPa or more.