JP5699764B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to an galvannealed steel sheet and a method for producing the same. More specifically, the present invention is an alloying and melting suitable for applications requiring excellent bendability, stretch flangeability and ductility in addition to high strength, such as reinforcement members for automobile undercarriage parts and members. The present invention relates to a galvanized steel sheet and a method for manufacturing the same.

  Recently, in order to protect the global environment, there has been a demand for improvement in fuel efficiency of automobiles. For automobile steel sheets, there is a need for high-strength steel sheets having a tensile strength (TS) of 780 MPa or more for the purpose of reducing vehicle weight and ensuring safety. Is growing. Such a steel plate is required to have not only high strength but also various characteristics. For example, from the viewpoint of formability, bendability, stretch flangeability, ductility, and the like are required. Moreover, the steel plate which gave hot-dip plating is calculated | required from a viewpoint of rust prevention.

  In general, steel strengthening methods include solid solution strengthening, precipitation strengthening, and transformation strengthening, and by combining these, the intended tensile strength can be achieved. However, if these combinations are different, steel sheets having different bendability, stretch flangeability, ductility, etc. can be obtained even if the tensile strength is the same. Therefore, in order to balance various required performances to a high degree, it is important to properly balance the strengthening methods.

Of the above-described strengthening methods, when transformation strengthening is used, it is possible to achieve high strength relatively easily.
For example, in the method for producing an alloyed hot-dip galvanized steel sheet described in Patent Document 1, a large amount of Si, Mn, Cr, and Mo is added, and the cooling rate is controlled, whereby a mixed structure of ferrite, bainite, and martensite is obtained. And TS ≧ 780 MPa is achieved.

Patent Document 2 describes that bendability and high strength can be achieved by obtaining tempered martensite.
According to these techniques, the strength can be increased relatively easily by using a mixed structure of ferrite and a hard phase.

  However, when a mixed structure containing a large amount of martensite is used, there is a drawback that cracks are likely to occur from the interface between the structures due to the large hardness difference between the structures, and the stretch flangeability and bendability are poor.

  Precipitation strengthening is often used as a method for increasing strength while suppressing deterioration in stretch flangeability and bendability. When precipitation strengthening is used, Ti and Nb are often added, but it is common to add a large amount of Ti which is inexpensive and has a large increase in strength with respect to the addition amount. . Furthermore, since the addition of Ti has the effect of refining the ferrite grains, there is an advantage that high strength can be achieved by refining the ferrite grains in addition to precipitation strengthening by Ti carbonitride.

Patent Document 3, Patent Document 4, and Patent Document 5 are cited as conventional techniques for high-tensile hot-dip galvanized steel sheets using precipitation strengthening by Ti.
However, when Ti is added and the refinement of ferrite or precipitation strengthening is used, the ferrite itself becomes very hard and the ductility tends to deteriorate significantly.

  Thus, the prior art has not yet provided an alloyed hot-dip galvanized steel sheet having high tensile strength and excellent bendability, stretch flangeability and ductility.

JP-A-4-173946 JP-A-6-108152 JP-A-6-322479 JP 2002-161336 A JP 2003-231941 A

  The present invention has been made in view of the above prior art, and is suitable as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and suspension parts, An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having high tensile strength and excellent bendability, stretch flangeability and ductility, and a method for producing the same.

The present inventors have intensively studied to solve the above problems.
As a result, while optimizing component segregation and surface shape (surface crack density) in the vicinity of the surface of the steel sheet to be the plating base material, by optimizing the steel structure, it is not always necessary to contain Ti or Nb, The inventors obtained new knowledge that an alloyed hot-dip galvanized steel sheet having excellent bendability, stretch flangeability and bendability can be obtained while having a high tensile strength of 780 MPa or more.

The present invention completed based on the above findings is as follows.
(1) An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet,
The steel plate
In mass%, C: 0.03% to 0.35%, Si: 0.005% to 2.0%, Mn: 1.0% to 4.0%, P: 0.0004% or more 0.1% or less, S: 0.02% or less, sol.Al: 0.0002% or more and 2.0% or less, N: 0.01% or less, balance Fe and chemical composition consisting of impurities,
The concentrated portion average interval which is the average interval in the direction perpendicular to the rolling direction of the concentrated portion where Mn and / or Si expanded in the rolling direction at a position of a depth of 50 μm from the surface of the steel sheet is 1000 μm or less,
The number density of cracks having a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet is 3 pieces / mm or more and 1000 pieces / mm or less,
The average value of the closest distances of martensite and retained austenite, including area percent, bainite: 60% or more, retained austenite: 1% or more, martensite: 1% or more, and ferrite: 2% or more but less than 20% Having an ultra-hard phase average interval of 20 μm or less,
The alloyed hot-dip galvanized steel sheet has mechanical properties having a tensile strength (TS) of 780 MPa or more.

(2) The galvannealed steel sheet according to (1), wherein the chemical composition further contains Bi: 0.5% by mass or less, and the concentrated part average interval is 500 μm or less.
(3) The chemical composition is, by mass, Ti: 1.0% or less, Nb: 1.0% or less, V: 1.0% or less, W: 1.0% or less, Cr: 1.0% Hereinafter, one or more selected from the group consisting of Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0%, and B: 0.01% or less are further contained. The galvannealed steel sheet according to (1) or (2).

  (4) The chemical composition is selected from the group consisting of REM: 0.1% or less, Mg: 0.05% or less, Ca: 0.05% or less, and Zr: 0.05% or less in mass%. The alloyed hot-dip galvanized steel sheet according to any one of the above (1) to (3), which further contains one kind or two or more kinds.

(5) The galvannealed steel sheet according to any one of (1) to (4), wherein the average interval between the super hard phases is 10 μm or less.
The method for producing an galvannealed steel sheet according to any one of the above (1) to (5), which comprises the following steps (A) to (E):
(A) a soluble steel casting step of casting under conditions that the average cooling rate of temperature range is 10 ° C. / sec or more from the slab surface to the solidus temperature of the liquidus temperature at the depth position of 10 mm;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 800 ° C or higher, and cooled at an average cooling rate of 10 ° C / second or higher, and 300 ° C or higher. A hot rolling step in which a hot rolled steel sheet is wound up in a temperature range of less than 580 ° C;
(C) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling treatment under conditions satisfying the following formula (1) to form a pickled steel sheet;
(D) a cold rolling step in which the pickled steel plate obtained by the pickling step is subjected to cold rolling at a reduction rate of 20% or more to obtain a cold rolled steel plate; and (E) obtained by the cold rolling step. After holding the cold-rolled steel sheet in a temperature range of 750 ° C. or higher and 1000 ° C. or lower for 5 seconds or longer and 1000 seconds or shorter, an average cooling rate of 2 ° C./second or higher and 70 ° C./second or lower to a temperature range of 300 ° C. or higher and 580 ° C. or lower Cool and hold in this temperature range for 2 seconds or more, then apply hot dip galvanization to obtain a hot dip galvanized steel sheet, and apply an alloying treatment to hold it in a temperature range of 700 ° C. or lower for 120 seconds or less. Continuous galvanizing process.

5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 2000000 (1)
The method for producing an galvannealed steel sheet according to any one of the above (1) to (5), comprising the following steps (a) to (f):
(A) a soluble steel, cast under the condition that the average cooling rate of temperature range is 10 ° C. / sec or more from the slab surface to the solidus temperature of the liquidus temperature at the depth position of 10mm casting process;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 800 ° C or higher, and cooled at an average cooling rate of 10 ° C / second or higher, and 300 ° C or higher. A hot rolling step in which a hot rolled steel sheet is wound up in a temperature range of less than 580 ° C;
(C) A pickling process in which the hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling treatment under the conditions satisfying the above formula (1) to obtain a pickled steel sheet;
(D) a cold rolling step in which the pickled steel plate obtained by the pickling step is subjected to cold rolling at a reduction rate of 20% or more to obtain a cold rolled steel plate;
(E) The cold-rolled steel sheet obtained by the cold rolling step is held at a temperature range of 750 ° C. or higher for 5 seconds or longer, and then at an average cooling rate of 2 ° C./second or higher and 200 ° C./second or lower at 250 ° C. or higher and 580 ° C. A pre-annealing step in which pre-annealing is performed for cooling to a temperature range of ≦ C ° C .; and (f) the cold-rolled steel sheet obtained by the pre-annealing step is held in a temperature range of 750 ° C. to 1000 ° C. for 5 seconds to 1000 seconds. After that, it is cooled to a temperature range of 300 ° C. or more and 580 ° C. or less at an average cooling rate of 2 ° C./second or more and 70 ° C./second or less and held in this temperature range for 2 seconds or more. A continuous hot-dip galvanizing step in which a steel sheet is subjected to an alloying treatment that is held for 120 seconds or less in a temperature range of 700 ° C. or lower to obtain an alloyed hot-dip galvanized steel sheet.

  Here, “the surface of the steel sheet” means an interface between the steel sheet as the plating base and the alloyed hot-dip galvanized layer in the alloyed hot-dip galvanized steel sheet. The surface of the steel sheet in the galvannealed steel sheet can be usually discriminated from the difference in contrast when the cross section of the galvannealed steel sheet is observed with reflected electrons (BSE image) using a scanning electron microscope. When the interface is unclear even in the reflected electrons (BSE image), the cross section of the alloyed hot dip galvanized steel sheet is subjected to surface analysis for elements contained in the alloyed hot dip galvanized layer, such as Fe, Al, Zn, etc. by EDX, The interface between the alloyed hot dip galvanized layer and the steel sheet is discriminated by setting the part where the Fe concentration is 70% by mass or more as the steel sheet and the part where the Fe concentration is less than 70% by mass as the alloyed hot dip galvanized layer.

  In the present invention, the minimum value of the inner radius at which cracking does not occur in the 180 ° bending test is 2.0 t or less (t: plate thickness) as the target characteristic of excellent bendability, and the tensile strength (TS) The product with a hole expansion rate (HER) (TS x HER value) of 28000 (MPa%) or more is the target characteristic of excellent stretch flangeability, and the tensile strength (TS) and total elongation (El) The product characteristic (TS × El value) of 10000 (MPa ·%) or more is regarded as an excellent ductility target characteristic.

  According to the present invention, an alloyed hot-dip galvanized steel sheet having excellent bendability, stretch flangeability and ductility while having a high strength of TS of 780 MPa or more can be obtained. The galvannealed steel sheet according to the present invention is suitable as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and suspension parts.

1. Chemical composition The chemical composition of the galvannealed steel sheet of the present invention will be described. “%” For chemical composition means “mass%”.

(1) C: 0.03% or more and 0.35% or less C has an action of generating a hard phase such as bainite, martensite, and retained austenite and improving the strength of the steel sheet. In particular, in the present invention, the area ratio of bainite needs to be 60% or more in the steel structure. If the C content is less than 0.03%, it is difficult to make the area ratio of bainite 60% or more. Therefore, the C content is 0.03% or more. Preferably it is 0.05% or more. On the other hand, when the C content exceeds 0.35%, the weldability is significantly lowered. Therefore, the C content is 0.35% or less. Preferably it is 0.25% or less.

(2) Si: 0.005% or more and 2.0% or less Si has an effect of increasing the strength of the steel sheet by solid solution strengthening. If the Si content is less than 0.005%, it is difficult to obtain the effect by the above action. Therefore, the Si content is set to 0.005% or more. Preferably it is 0.01% or more. On the other hand, if the Si content exceeds 2.0%, the wettability with hot dip galvanizing deteriorates and there are many unplated parts, and the corrosion resistance deteriorates significantly. Therefore, the Si content is 2.0% or less. Preferably it is 1.8% or less.

(3) Mn: 1.0% or more and 4.0% or less Mn has the effect of increasing the strength of the steel sheet by increasing the hardenability of the steel. If the Mn content is less than 1.0%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 1.0% or more. Preferably it is 1.2% or more. On the other hand, if the Mn content exceeds 4.0%, the hardenability is excessively increased, the area ratio of martensite is excessive, and stretch flangeability and bendability are significantly deteriorated. Therefore, the Mn content is 4.0% or less. Preferably it is 3.0% or less.

(4) P: 0.0004% or more and 0.1% or less P has an effect of increasing the strength of the steel sheet by solid solution strengthening. If the P content is less than 0.0004%, it is difficult to obtain the effect by the above action. Therefore, the P content is set to 0.0004% or more. Preferably it is 0.006% or more. On the other hand, since P is an element that easily segregates, a large amount thereof causes a decrease in weldability. If the P content exceeds 0.1%, the weldability is significantly reduced due to segregation. Therefore, the P content is 0.1% or less. Preferably it is 0.08% or less.

(5) S: 0.02% or less S is contained as an impurity, and has the effect of forming sulfides in the steel to reduce stretch flangeability and bendability. If the S content exceeds 0.02%, the stretch flangeability and the bendability deteriorate significantly. Therefore, the S content is 0.02% or less. Preferably it is 0.01% or less. The lower the S content, the better. Therefore, the lower limit of the S content need not be specified, but from the viewpoint of steelmaking cost, it is preferably set to 0.0002% or more.

(6) sol.Al: 0.0002% or more and 2.0% or less Al has a function of deoxidizing steel and making the steel plate sound. If the sol.Al content is less than 0.0002%, it is difficult to obtain the effect by the above action. Therefore, the sol.Al content is 0.0002% or more. Preferably it is 0.0005% or more. On the other hand, if the sol.Al content exceeds 2.0%, coarse alumina inclusions increase, and the stretch flangeability and bendability are significantly reduced. Therefore, the sol.Al content is 2.0% or less. Preferably it is 1.8% or less.

(7) N: 0.01% or less N is contained as an impurity, and has the effect of forming nitrides in the steel to reduce stretch flangeability and bendability. If the N content exceeds 0.01%, the bendability will be significantly reduced. Therefore, the N content is 0.01% or less. Preferably it is 0.008% or less. The lower the N content, the better. Therefore, it is not necessary to define the lower limit of the N content, but from the viewpoint of steelmaking cost, it is preferably set to 0.0002% or more.

The elements described below are optional elements that may be contained in the steel in some cases.
(8) Bi: 0.5% or less Bi serves as an inoculation nucleus for coagulation, and has the effect of reducing the interval between dendritic arms during coagulation and making the coagulated tissue finer. As a result, segregation of elements such as Mn and Si that are likely to be segregated is suppressed, the local strength difference of the steel sheet is reduced, and the bendability is improved. Therefore, it is preferable to contain Bi. However, Bi forms an oxide in the steel that becomes the starting point of cracking during bending, and therefore the bendability deteriorates significantly when the Bi content exceeds 0.5%. Therefore, the Bi content when Bi is contained is 0.5% or less. Preferably it is 0.03% or less. In order to more reliably obtain the effect of the above action, the Bi content is preferably set to 0.0002% or more. By doing so, at a position of a depth of 50 μm from the interface between the hot dip galvanized layer and the steel plate. It is possible to more surely make the average interval (details will be described later) in the direction perpendicular to the rolling of the thickened portion where Mn and / or Si concentrated in the rolling direction is 500 μm or less.

(9) Ti: 1.0% or less, Nb: 1.0% or less, V: 1.0% or less, W: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less Cu, 1.0% or less, Ni: 1.0%, and B: one or more selected from the group consisting of 0.01% or less Ti, Nb, V, W, Cr, Mo, Cu , Ni and B have the effect of increasing the strength of the steel sheet by increasing the hardenability of the steel, similar to Mn. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when Ti, Nb, V, W, Cr, Mo, Cu and Ni have a content of more than 1.0%, and B has a content of more than 0.01%, the hardenability is excessive. The area ratio of martensite becomes excessive and the stretch flangeability and bendability deteriorate significantly. Therefore, the contents of Ti, Nb, V, W, Cr, Mo, Cu, Ni, and B when contained are as described above. In order to more reliably obtain the effect of the above action, the element of any one of Ti, Nb, V, W, Cr, Mo, Cu and Ni is set to 0.005% or more, or the B content is set to 0.0002. % Or more is preferable. More preferably, the content of B is 0.0004% or more.

(10) One or more selected from the group consisting of REM: 0.1% or less, Mg: 0.05% or less, Ca: 0.05% or less, and Zr: 0.05% or less REM ( Rare earth elements), Mg, Ca and Zr have the effect of improving the stretch flangeability and bendability by finely spheroidizing oxides and sulfides formed in steel. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when the content of REM exceeds 0.1%, the content of Mg, Ca and Zr exceeds 0.05%, respectively, the number of oxides and sulfides formed in the steel becomes excessive. Deteriorates stretch flangeability and bendability. Accordingly, the contents of REM (rare earth element), Mg, Ca, and Zr in the case of inclusion are as described above. In order to more reliably obtain the effect of the above action, the content of any one of REM, Mg, Ca and Zr is preferably set to 0.0002% or more.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

2. Concentrated portion, surface shape and steel structure of steel plate The concentrated portion, surface shape and steel structure of the steel plate which is the plating base material of the galvannealed steel plate of the present invention will be described.

(1) Concentrated portion average interval which is an average interval in the direction perpendicular to the rolling direction of the concentrated portion of Mn and / or Si expanded in the rolling direction at a depth of 50 μm from the surface of the steel plate: 1000 μm or less To 50 μm depth (hereinafter also referred to as “depth position A”), which is expanded in the rolling direction and enriched with Mn and / or Si concentration (hereinafter referred to as “Mn / Si concentration”). (Also referred to as “concentrated portion average spacing” in the present invention) in the direction perpendicular to the rolling direction, that is, the width direction of the steel sheet (referred to as “the rolling perpendicular direction” in the present invention). )) Of 1000 μm or less, good bendability can be obtained.

  Here, the Mn / Si concentrated part is defined as a site where the content of at least one element of Mn and Si is 1.1 times or more with respect to the bulk content (average content of the steel sheet). The Mn / Si enriched portion in the cold-rolled steel sheet is formed by extending the segregated Mn and Si in the slab in the rolling direction during hot rolling and cold rolling.

  The method for obtaining the thickened part average interval is as follows. First, the surface of the galvannealed steel sheet is ground to expose the surface at the depth position A. EPMA line analysis is performed on the exposed surface in the direction perpendicular to the rolling direction. The measurement distance by one line analysis is preferably 3 mm or more so that an average interval of 1000 μm can be measured. For each of the line profiles of the Si concentration and the Mn concentration obtained by line analysis, the average concentration is obtained, and this concentration is taken as the bulk content. Regions in which the Si concentration and / or Mn concentration in the line profile is 1.1 times the average concentration are obtained, and these regions are defined as Mn / Si enriched portions. The portion showing the maximum density in each area constituting the obtained thickened portion is set as the center point of the area. The distance between the center points of adjacent regions is obtained, and these are averaged within the line profile, and the obtained average value is taken as the thickened portion average interval.

  When the average interval between the enriched parts exceeds 1000 μm, the concentration of Mn and Si is unevenly generated, so that local hardness variation due to the concentration of components occurs in the steel sheet. When bending is performed on such an alloyed hot-dip galvanized steel sheet, it is easy to produce a working stripe on the surface of the alloyed hot-dip galvanized steel sheet. Stress concentration occurs in the recesses forming the machining streaks, initial cracks due to bending work occur early, and bendability deteriorates. Therefore, in order to improve the bendability, it is necessary to suppress the occurrence of machining streaks by suppressing segregation of Mn and Si and uniformly dispersing them. Therefore, the thickened portion average interval is set to 1000 μm or less. Preferably it is 500 micrometers or less.

  A preferable thickened part average interval of 500 μm or less can be achieved more reliably by containing Bi as described above. The lower the concentrated portion average interval, the better. The lower limit is not particularly specified, but the temperature from the liquidus temperature to the solidus temperature is taken into consideration when casting a slab of about 30 mm to 350 mm, which is usually a slab thickness. From the relationship of the average cooling rate in the region, it is practically preferable that the thickening portion average interval be 3 μm or more.

  In addition, the position where the concentrated portion average interval is measured is set to the depth position A because the outermost layer portion of the steel plate affected by the work alteration is excluded, and the Mn in the steel plate surface portion affecting the bendability This is to properly evaluate the concentration of Si and Si.

(2) Number density of cracks having a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet: 3 pieces / mm or more and 1000 pieces / mm or less Number of cracks having a depth of 3 μm or more and 10 μm or less on the steel sheet surface (hereinafter “surface”) A good bendability can be obtained by setting the number of cracks to 3 / mm to 1000 / mm.

  By forming appropriate cracks on the surface of the steel plate, stress concentration in bending can be dispersed and stress concentration can be suppressed, so that good bendability can be obtained. Here, the crack on the steel sheet surface (hereinafter referred to as “surface crack”) means a crack opened on the steel sheet surface.

  If the depth of the surface crack is less than 3 μm, the effect of suppressing the stress concentration is small. On the other hand, if the depth of the surface crack exceeds 10 μm, there is a possibility that the crack itself becomes a starting point of the crack in the bending process, and the bendability may be deteriorated on the contrary. Further, if the surface crack number density of 3 μm or more and less than 10 is less than 3 / mm, the effect of suppressing the stress concentration may not be sufficiently obtained. On the other hand, if the surface crack number density exceeds 1000 / mm, the cracks are likely to be connected to each other at the time of bending, and the possibility of developing into a large crack becomes the starting point of the crack, and the bendability may be deteriorated on the contrary. is there. Accordingly, the number density of surface cracks having a depth of 3 μm or more and 10 μm or less is set to 3 pieces / mm or more and 1000 pieces / mm or less.

By doing in this way, the bendability which was excellent in combination with the prescription | regulation of the thickening part average space | interval mentioned above can be obtained.
The surface crack number density may be measured as follows. That is, a cross section of the galvannealed steel sheet is observed, and a crack having a depth of 3 μm or more and 10 μm or less is specified. Count the number of these cracks identified in the field of view. The interface observed linearly in the observed image is linearly approximated, and the crack number density is obtained by dividing the number of cracks counted by the length of the straight line in the observation field.

(3) Bainite: 60 area% or more, retained austenite: 1 area% or more, martensite: 1 area% or more, ferrite: 2 area% or more but less than 20 area%, average value of closest distance of martensite and retained austenite ( (Super hard phase average interval): 20 μm or less Bainite: 60 area% or more Bainite is a hard structure and a uniform hardness distribution. For this reason, it is the most effective structure for ensuring high strength and good strength-stretch flangeability balance. When the area ratio of bainite is less than 60%, it is difficult to ensure high strength and excellent strength-stretch flangeability balance of TS: 780 MPa or more and TS × HER value: 28000 MPa ·% or more. Therefore, the area ratio of bainite is 60% or more. Preferably it is 65% or more.

Residual austenite: 1 area% or more Residual austenite is a phase that dramatically improves the ductility of the steel sheet due to the transformation-induced plasticity (TRIP) effect. As described above, bainite is the most effective structure for securing a high strength and a good strength-stretch flangeability balance, but is a structure having poor ductility. Since bainite, which is a structure inferior in ductility, is contained in an area of 60% by area or more, in order to ensure an excellent strength-ductility balance, the ductility of the steel sheet is improved by the transformation-induced plasticity (TRIP) effect of retained austenite. There is a need. If the area ratio of retained austenite is less than 1%, sufficient transformation-induced plasticity (TRIP) effect cannot be obtained, and it is difficult to secure an excellent strength-ductility balance of 10000 MPa ·% or more in terms of TS × El value. Become. Therefore, the area ratio of retained austenite is 1% or more. Preferably it is 2% or more.

Martensite: 1 area% or more Martensite is a very hard phase and is a phase that dramatically improves the strength of the steel sheet. As described above, since bainite is a hard structure, it is an effective structure for securing high strength, but is softer than martensite. For this reason, it is difficult to ensure a very high strength only by bainite. In order to ensure a very high strength, it is necessary to improve the strength of the steel sheet by containing martensite. If the area ratio of martensite is less than 1%, it is difficult to ensure a high strength of TS: 780 MPa or more. Therefore, the area ratio of martensite is 1% or more. Preferably it is 2% or more.

Ferrite: 2 area% or more and less than 20 area% Ferrite is the softest phase and is a phase that improves the ductility of the steel sheet. As described above, bainite is the most effective structure for securing a high strength and a good strength-stretch flangeability balance, but is a structure having poor ductility. Thus, since bainite which is a structure | tissue inferior to ductility is contained 60 area% or more, in order to ensure the outstanding strength-ductility balance, it is necessary to contain a ferrite and to improve the ductility of a steel plate. When the area ratio of ferrite is less than 1%, the effect of improving ductility by ferrite cannot be sufficiently obtained, and it becomes difficult to ensure an excellent strength-ductility balance of 10000 MPa ·% or more in terms of TS × El value. Therefore, the area ratio of ferrite is 1% or more. Preferably it is 3% or more.

  On the other hand, since ferrite is the softest phase, it is also a phase that makes it difficult to improve the strength of the steel sheet. If the ferrite area ratio is 20% or more, it is difficult to ensure a high strength of TS: 780 MPa or more. Therefore, the area ratio of ferrite is less than 20%. Preferably it is 18% or less.

Average value of closest distance between martensite and retained austenite (super hard phase average interval): 20 μm or less As described above, martensite has the effect of improving the strength of the steel sheet, and retained austenite improves the ductility of the steel sheet. Since it has an effect | action, in order to ensure a high intensity | strength and the outstanding strength-ductility balance, all are made to contain 1 area% or more.

  However, all of these phases have the effect of stretching and reducing the flangeability. That is, when stretched flange forming is applied to a steel sheet, martensite is very hard, whereas bainite is softer than martensite and ferrite is remarkably softer than martensite. For this reason, stress concentration occurs at the interface between martensite and bainite and the interface between martensite and ferrite due to these hardness differences, and cracks occur early. In addition, when retained austenite is stretched and flange-formed on a steel sheet, it becomes martensite due to strain-induced transformation, and similarly cracks occur early.

Therefore, it is difficult to ensure an excellent strength-stretch flangeability balance simply by containing these phases.
Therefore, by dispersing finely martensite and retained austenite (hereinafter sometimes collectively referred to as “ultra-hard phase”), it is possible to effectively disperse the stress at the time of stretch flange deformation, resulting from the above hardness difference. By controlling the stress concentration, excellent strength-stretch flangeability balance is ensured.

  When the average value of the closest distance of martensite and retained austenite (super hard phase average interval) exceeds 20 μm, effective stress distribution cannot be achieved, and excellent strength-elongation of TS × HER value: 28000 MPa ·% or more. It is difficult to ensure a flange balance. Accordingly, the average interval between the super-hard phases is set to 20 μm or less. Preferably it is 10 micrometers or less.

  Here, the average value of the closest distance between the martensite and retained austenite (superhard phase average interval) may be obtained as follows. In other words, the cross-section parallel to the rolling direction of the galvannealed steel sheet is subjected to night etching to obtain a sample for cross-sectional observation. About the obtained sample, a steel structure is observed using a scanning electron microscope. The measurement magnification is 1000 times, and 10 fields of view and 20 fields of view are observed at positions at a depth of 1/4 of the plate thickness (hereinafter referred to as “plate thickness 1/4 position”) from both surfaces of the steel plate. Martensite and retained austenite, which are superhard phases, are specified for all of the obtained 20-view steel structure images. For the identified superhard phase, the distance to the nearest other superhard phase (hereinafter referred to as “closest distance”) is measured. By selecting the longest and shortest closest measured distances for each field of view, the 20 longest closest distances and the 20 shortest closest distances are determined. The arithmetic average value in the data of these 40 closest distances is taken as the average value (superhard phase average interval) of the closest distances of martensite and retained austenite.

  In addition, the ferrite here is a phase specified as ferrite such as polygonal ferrite and bainitic ferrite. Moreover, bainite is a structure described as bainite such as lower bainite and upper bainite. As long as the above-mentioned area ratio is satisfied for each phase, the steel structure may include a phase or structure other than the above, for example, a structure such as pearlite.

3. Alloyed hot-dip galvanized layer The alloyed hot-dip galvanized layer formed on the surface of the steel sheet is not particularly defined, but is preferably as follows. The alloyed hot dip galvanized layer is generally formed on both surfaces of a steel plate which is a plating base. However, this invention includes the case where it forms only on one side.

That is, coating weight (per side) is preferably be from the viewpoint of corrosion resistance 3 g / m ² or more, still more preferably 10 g / m ² or more. Further, from the viewpoint of preventing defects such as blow holes during welding, it is preferably 200 g / m ² or less, and more preferably 100 g / m ² or less.

  The Fe concentration in the plating layer is preferably 3% by mass or more and 20% by mass or less from the viewpoint of plating adhesion and powdering properties. By setting the Fe concentration in the plating layer to 3% by mass or more, the adhesion of plating by alloying can be further improved. Moreover, favorable powdering property is securable by making Fe density | concentration in a plating layer into 20 mass% or less. The Fe concentration in the plating layer is more preferably 7% by mass or more and 16% by mass or less.

  In the alloyed hot dip galvanized layer, there is a possibility that alloy elements such as Si, Mn, P, and S may be taken in from the steel plate as the plating base material during the alloying treatment, but under normal conditions If it is within the range of the amount incorporated in the alloyed hot-dip galvanized layer during the hot dipping and alloying treatment, the plating quality will not be adversely affected.

4). Manufacturing method Next, the manufacturing method of the galvannealed steel plate of this invention is demonstrated for every process.
(1) Casting process Conditions for the molten steel having the above chemical composition to have an average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface to 10 ° C / second or more. Cast with

The average cooling rate greatly affects the segregation of Mn and Si.
When the average cooling rate is less than 10 ° C./second, the solidification rate is too low, so that the dendrite arm interval in the slab is widened, and Mn and N in the rolling direction at a depth of 50 μm (depth position A) from the surface of the steel sheet. It becomes difficult to make the average interval (concentrated portion average interval) in the direction perpendicular to the rolling of the Mn / Si enriched portion where Si is expanded / 1000 μm or less. Therefore, the average cooling rate is 10 ° C./second or more. Preferably, it is 12 ° C./second or more.

As described above, when Bi is contained, in combination with the effect of making the solidified structure fine by Bi, it is possible to more surely realize the concentrated portion average interval of 500 μm or less.
(2) Hot rolling step The slab obtained by the above casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 800 ° C or higher, and cooled at an average cooling rate of 10 ° C / second or higher. The steel sheet is wound in a temperature range of 300 ° C. or higher and lower than 580 ° C. to obtain a hot rolled steel sheet.

In order to make the steel structure that is the plating base of the galvannealed steel sheet into the steel structure described above, it is important that the steel structure of the hot-rolled steel sheet contains 60% by area or more of bainite.
That is, the bainite in the hot-rolled steel sheet is then divided into ferrite and cementite by cold rolling. For this reason, the cementite derived from the bainite in a hot-rolled steel sheet exhibits a densely dispersed form after cold rolling. The densely dispersed cementite then becomes the core of austenitization in the soaking process in the continuous hot dip galvanizing process described later, and austenitization proceeds, and the subsequent cooling becomes bainite, martensite, residual austenite, and the like. Here, since the distribution state of martensite and retained austenite after cooling drags the influence of the distribution state of nuclei for austenitization, the average interval between superhard phases, which is the average value of the closest distance between martensite and retained austenite, is 20 μm or less. The steel structure which is can be obtained.

  And in order to make the steel structure of a hot-rolled steel sheet contain 60 area% or more of bainite, the hot rolling completion temperature needs to be 800 degreeC or more. If the hot rolling completion temperature is less than 800 ° C., the ferrite transformation proceeds early, and it is difficult to make the area ratio of bainite 60% or more. Therefore, the hot rolling completion temperature is 800 ° C. or higher. From the viewpoint of securing the steel structure, it is not necessary to define the upper limit of the hot rolling completion temperature. However, when the hot rolling completion temperature becomes excessively high, the scale may be formed thick, which may induce surface defects. Therefore, from the viewpoint of suppressing the occurrence of surface defects due to scale, it is preferable that the hot rolling completion temperature is 1000 ° C. or less.

  If the average cooling rate in cooling from completion of hot rolling to winding is less than 10 ° C./second, ferrite transformation proceeds excessively due to the cooling rate being too low, and the area ratio of bainite is 60% or more. It becomes difficult to do. Therefore, the average cooling rate is 10 ° C./second or more. Preferably, it is 15 ° C./second or more. From the viewpoint of securing the steel structure, it is not necessary to define the upper limit of the average cooling rate. However, if the average cooling rate is excessively high, uneven cooling becomes obvious, uneven unevenness due to uneven cooling becomes remarkable, and trouble may occur during cold rolling. Therefore, from the viewpoint of ensuring good cold rollability, the average cooling rate is preferably 500 ° C./second or less.

  If the coiling temperature is less than 300 ° C., the steel structure of the hot-rolled steel sheet contains martensite in an area of 40% by area or more, and therefore the bainite area ratio cannot be made 60% or more. Therefore, the coiling temperature is set to 300 ° C. or higher. Preferably it is 350 degreeC or more. On the other hand, when the coiling temperature is 580 ° C. or higher, the pearlite transformation proceeds, the steel structure of the hot-rolled steel sheet is composed of ferrite and pearlite, and the area ratio of bainite is 60% or more. Can not. Therefore, the coiling temperature is less than 580 ° C. Preferably it is 550 degrees C or less.

(3) Pickling process The pickled steel sheet is obtained by subjecting the hot-rolled steel sheet obtained by the hot rolling process to a pickling process under conditions that satisfy the following formula (1).

5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 2000000 (1)
The surface crack of the cold-rolled steel sheet is formed by selectively oxidizing the ferrite grain boundary portion by pickling after hot rolling. If the value of acid concentration (mass%) x acid temperature (° C) x acid soaking time (seconds) is less than 5000, selective oxidation of ferrite grain boundaries becomes insufficient, and the surface of the steel sheet (interface with the plating layer) It is difficult to make the number density of surface cracks having a depth of 3 μm or more and 10 μm or less at 3 / mm or more. On the other hand, when the value of acid concentration (mass%) × acid temperature (° C.) × acid dipping time (seconds) exceeds 2000000, selective oxidation of ferrite grain boundaries proceeds excessively, and the surface of the steel sheet It becomes difficult to set the number density of surface cracks having a depth of 3 μm or more and 10 μm or less to 1000 pieces / mm or less. The value of acid concentration (mass%) × acid temperature (° C.) × acid immersion time (seconds) is preferably 10,000 or more and less than 1900000.

In addition, the kind of acid used at a pickling process is not specifically limited, Hydrochloric acid and a sulfuric acid are illustrated.
(4) Cold rolling process Cold-rolled steel sheet is obtained by subjecting the pickled steel sheet obtained by the above pickling process to cold rolling with a rolling reduction of 20% or more.

  If the rolling reduction in cold rolling is less than 20%, the rolling reduction is too low, so that the bainite produced from the hot-rolled steel sheet is not divided into fine ferrite and cementite by rolling and is plated after the subsequent continuous hot dip galvanizing process. The steel structure of the steel sheet as the base material cannot have martensite, retained austenite, and an average process interval within 20 μm. Therefore, the rolling reduction during cold rolling is 20% or more. Preferably it is 30% or more. From the viewpoint of dividing the bainite of the hot-rolled steel sheet, the higher the reduction ratio in cold rolling, the better, so it is not necessary to define the upper limit. However, as the rolling reduction increases, the rolling load increases and operation becomes difficult. Therefore, from the viewpoint of rolling load, the rolling reduction is preferably 90% or less.

(5) Pre-annealing process The cold-rolled steel sheet obtained by the cold rolling process can be directly subjected to hot dip galvanizing and alloying heat treatment in the continuous hot dip galvanizing process described below. As a process, after holding the cold-rolled steel sheet in a temperature range of 750 ° C. or higher for 5 seconds or more, before cooling to a temperature range of 250 ° C. or higher and 580 ° C. or lower at an average cooling rate of 2 ° C./second or higher and 200 ° C./second or lower. It is preferable to perform annealing.

  By performing the above-mentioned pre-annealing before hot dip galvanization, it becomes possible to make the bainite in the hot rolled steel sheet a more dense structure, and thereby, the steel structure of the steel sheet that is the plating base of the galvannealed steel sheet The average value of the closest distances of martensite and retained austenite can be set to a preferable value of 10 μm or less.

  In order to make the bainite in the hot-rolled steel sheet more dense by pre-annealing, in the pre-annealing process, as in the continuous hot-dip galvanizing process to be described later, soaking treatment is performed to keep the cold-rolled steel sheet in a high temperature range. It is necessary to once austenite part or all of the steel structure of the cold-rolled steel sheet.

  Here, when the soaking temperature in soaking is less than 750 ° C., austenitization is insufficient, so that it is difficult to make the bainite in the hot-rolled steel sheet into a more dense structure. Therefore, the soaking temperature is preferably 750 ° C. or higher. The upper limit of the soaking temperature is not particularly specified, but is preferably set to 1000 ° C. or less from the viewpoint of suppressing damage to the refractory in the annealing furnace.

  Further, if the soaking time in soaking is less than 5 seconds, austenitization is insufficient, so that it is difficult to make the bainite in the hot-rolled steel sheet a more dense structure. Therefore, the soaking time is preferably 5 seconds or longer. A more preferable soaking time is 30 seconds or more. The upper limit of the soaking time does not need to be specified, but is preferably 1000 seconds or less from the viewpoint of productivity.

  When the average cooling rate after the soaking is less than 2 ° C./second, it is difficult to make the bainite in the hot-rolled steel sheet a denser structure. Therefore, the average cooling rate is preferably 2 ° C./second or more. More preferably, it is 10 ° C./second or more. On the other hand, when the average cooling rate exceeds 200 ° C./second, martensite is excessively generated because the cooling rate is too high, and it becomes difficult to make the bainite in the hot-rolled steel sheet into a denser structure. There is. Therefore, the average cooling rate is preferably 200 ° C./second or less.

  If the cooling stop temperature is less than 250 ° C., martensite is excessively generated, and it may be difficult to make the bainite in the hot-rolled steel sheet into a more dense structure. Therefore, the cooling stop temperature is preferably 250 ° C. or higher. More preferably, it is 300 degreeC or more. On the other hand, when the cooling stop temperature exceeds 580 ° C., it may be difficult to generate bainite. Therefore, the cooling stop temperature is preferably 580 ° C. or lower. More preferably, it is 550 degrees C or less.

(6) Continuous hot-dip galvanizing process The cold-rolled steel sheet obtained by the said cold rolling process or preferably the cold-rolled steel sheet obtained by the said pre-annealing process is 1000 to 1000 degreeC for 5 second or more 1000 degree | times. After holding for 2 seconds or less, cool to a temperature range of 300 ° C. or more and 580 ° C. or less at an average cooling rate of 2 ° C./s or more and 70 ° C./s or less, hold in this temperature range for 2 seconds or more, and then apply hot dip galvanization Thus, a hot dip galvanized steel sheet is obtained, and an alloying treatment is performed in a temperature range of 700 ° C. or lower for 120 seconds or less to obtain a galvannealed steel sheet.

  In order to make the steel sheet that is the plating base of the alloyed hot-dip galvanized steel sheet into the above steel structure, in the continuous hot-dip galvanizing process, by performing a soaking treatment that keeps the cold-rolled steel sheet in a high temperature range, Some or all of the tissue needs to be austenitized once.

  Here, when the soaking temperature in the soaking process is less than 750 ° C., the area of ferrite is 20% or more because it is a temperature range in which ferrite formation is active. Therefore, the soaking temperature is set to 750 ° C. or higher. Preferably it is 780 degreeC or more. On the other hand, when the soaking temperature exceeds 1000 ° C., the austenite grain size during soaking increases, ferrite is not generated, and the ferrite area ratio is less than 2%. Therefore, the soaking temperature is set to 1000 ° C. or less. Preferably it is 920 degrees C or less.

  Further, if the soaking time in soaking is less than 5 seconds, the pearlite, bainite and cementite generated during hot rolling cannot be sufficiently austenitized, and it is difficult to make the residual austenite area ratio 1% or more. It becomes. Therefore, the soaking time is 5 seconds or more. Preferably it is 20 seconds or more. On the other hand, when the soaking time exceeds 1000 seconds, the generation of ferrite proceeds excessively, and it may be difficult to make the ferrite area ratio less than 20%. Therefore, the soaking time is 1000 seconds or less. Preferably it is 800 seconds or less.

  In order to make the steel structure that is the plating base of the alloyed hot-dip galvanized steel sheet into the above steel structure, it is necessary to generate bainite, martensite and retained austenite from austenite by cooling and holding.

  Here, if the average cooling rate of cooling after soaking is less than 2 ° C./second, ferrite is excessively generated, and it is difficult to make the ferrite area ratio less than 20%. Therefore, the average cooling rate is 2 ° C./second or more. Preferably, it is 4 ° C./second or more. On the other hand, when the average cooling rate exceeds 70 ° C./second, the generation of ferrite becomes insufficient, and it may be difficult to make the ferrite area ratio 2% or more. Therefore, the average cooling rate is set to 70 ° C./second or less. Preferably it is 60 degrees C / sec or less.

  If the cooling end point temperature is less than 300 ° C., martensite is excessively generated, and it becomes difficult to make the area ratio of bainite 60% or more. Therefore, the cooling end point temperature is set to 300 ° C. or higher. Preferably it is 450 degreeC or more. On the other hand, when the cooling end point temperature exceeds 580 ° C., it is difficult to promote the formation of bainite and martensite. On the other hand, since the formation of ferrite proceeds excessively, the area ratio of bainite is 60% or more and It may be difficult to make the area ratio 1% or more, and the ferrite area ratio may be 20% or more. Therefore, the cooling end point temperature is set to 580 ° C. or lower. Preferably it is 560 degrees C or less.

  If the holding time in the temperature range of 300 ° C. or higher and 580 ° C. or lower after cooling is less than 2 seconds, it is difficult to promote the formation of residual austenite, and the area ratio of residual austenite may be 1.0% or higher. Have difficulty. Therefore, the time for maintaining the temperature range is 2 seconds or more. Preferably it is 4 seconds or more. Although the upper limit of the time kept in the temperature range is not particularly defined, it is preferably 300 seconds or less from the viewpoint of productivity.

After these treatments, an alloyed hot dip galvanized layer is formed on the surface of the steel sheet by performing hot dip galvanizing treatment and alloying treatment.
Here, when the holding temperature in the alloying treatment exceeds 700 ° C., the retained austenite is decomposed into cementite, and it becomes difficult to make the area ratio of the retained austenite 1.0% or more. Therefore, the holding temperature in the alloying process is set to 700 ° C. or less. The lower limit of the holding temperature in the alloying treatment is not particularly specified, but if it is less than 450 ° C., the diffusion of Fe and zinc is slow and the alloying does not proceed rapidly, so the productivity is hindered. Therefore, the holding temperature in the alloying treatment is preferably set to 450 ° C. or higher.

  If the holding time in the alloying process exceeds 120 seconds, the retained austenite may be decomposed and the area ratio of the retained austenite may be less than 1%. Therefore, the holding time in the alloying process is 120 seconds or less. The lower limit of the holding time in the alloying treatment is not particularly required as long as a sufficient alloying treatment is achieved.

  Thus, an alloyed hot-dip galvanized steel sheet having a steel structure, a concentrated portion near the surface, and a surface crack density defined in the present invention is obtained. The alloyed hot-dip galvanized steel sheet can be subjected to various surface treatments, for example, chemical conversion treatment, particularly non-chromium chemical conversion treatment, phosphate treatment, or lubrication treatment, as desired.

The present invention will be described more specifically with reference to examples.
Steels having the chemical components shown in Table 1 were melted in a converter and continuously cast using a continuous casting tester to obtain a slab having a width of 1000 mm and a thickness of 250 mm. At this time, the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a position 10 mm deep from the surface of the slab was changed by changing the amount of cooling water in the mold.

  The slab thus obtained was heated and hot-rolled with a hot rolling tester to obtain a hot-rolled steel sheet, and then subjected to a pickling treatment with hydrochloric acid to obtain a pickled steel sheet. Thereafter, cold rolling was performed to obtain a cold rolled steel sheet.

The cold-rolled steel sheet thus obtained was subjected to heat treatment, hot dip galvanizing treatment and alloying treatment using a continuous hot dip galvanizing tester. Some test materials were pre-annealed with a continuous annealing tester before being subjected to a continuous hot dip galvanizing tester. The plating adhesion amount was in the range of 20 to 150 g / m ² per side.

  These production conditions are shown in Table 2 (casting process to cold rolling process) and Table 3 (arbitrary pre-annealing process to continuous hot dip galvanizing process).

The following tests were conducted on the galvannealed steel sheets and slabs thus obtained.
(1) Average cooling rate in the temperature range from the liquidus temperature to the solidus temperature The average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the slab is picric acid in the cross section of the obtained slab Etching was performed, and the dendrite secondary arm interval λ (μm) was measured at 100 points at a 10 mm position in the depth direction from the slab surface at a 5 mm pitch in the casting direction. The cooling rate A (° C./second) in the temperature range from the temperature to the solidus temperature was calculated, and the average cooling rate was determined as an average value obtained by arithmetically averaging the 100 A values.

λ = 710 × A−0.39
(2) Average interval in the direction perpendicular to the rolling of the enriched portion of Mn and / or Si expanded in the rolling direction at a depth of 50 μm (depth position A) from the interface between the hot-dip plated layer and the steel sheet Average interval)
The measurement of the above-mentioned average of the concentrated part was carried out by EPMA line analysis. That is, it was ground from the interface to a depth of 50 μm, and EPMA line analysis was performed. The concentration of Mn and Si was determined by reading the waveform of the concentration of Si and Mn obtained from the line analysis, and determining from the interval between the concentration maximum values where the concentration average value of at least one of Si and Mn was 1.1 times or more.

  Specifically, the method for measuring the thickened portion average interval was as follows. That is, the surface of the plated steel sheet was ground to expose the surface at the depth position A. The exposed surface was subjected to EPMA line analysis in the direction perpendicular to the rolling direction. The measurement distance by one line analysis was set to 3 mm or more. For each of the Si and Mn concentration line profiles determined by line analysis, the average concentration was determined, and this concentration was taken as the bulk content. Regions where the Si concentration or Mn concentration in the line profile was 1.1 times the average concentration were determined, and these regions were designated as the enriched portions. The portion showing the maximum density in each of the regions constituting the obtained thickened portion was defined as the center point of that region. The distance between the center points of adjacent regions was determined, averaged within the line profile, and the average value obtained was defined as the thickened portion average interval.

(3) Evaluation of steel structure A sample for observing a cross section parallel to the rolling direction of a steel sheet was prepared by polishing and etching using a night liquid according to a conventional method, and using a scanning electron microscope, ferrite, bainite, martensite The area ratio of sites and retained austenite was determined by image processing.

  In addition, the average distance between the superhard phases, which is the average value of the closest distance between martensite and retained austenite, is measured by a scanning electron microscope with a magnification of 1000 times using a specimen for cross-sectional observation parallel to the rolling direction of the same steel sheet. By observing 10 visual fields and 20 visual fields at a thickness of 1/4 each from the surfaces of both surfaces, the following was obtained. That is, for each of the obtained 20 visual field steel structure images, the martensite and retained austenite, which are super hard phases, are specified, and the distance between the specified super hard phase and the other closest super hard phase is determined. Measure the closest distance. By selecting the longest and shortest closest measured distances for each field of view, the 20 longest closest distances and the 20 shortest closest distances are determined. The arithmetic average value in the data of these 40 closest distances is taken as the average value (superhard phase average interval) of the closest distances of martensite and retained austenite.

(4) Crack number density The crack number density is an observation sample having a cross section parallel to the rolling direction of the steel sheet used in the evaluation of the steel structure, and the cross section of this sample is 2000 times magnification using a scanning electron microscope. The number of surface cracks whose depth from the steel sheet surface was 3 μm or more and 10 μm or less, which was obtained by observing 100 fields of view, was calculated by converting into the number per unit length. Specifically, it was as follows.

  Specifically, the method for measuring the crack number density was as follows. That is, cross-sectional observation (observation magnification: 2000 times) of the plated steel sheet was performed, and surface cracks having a depth of 3 μm or more and 10 μm or less from the surface of the steel sheet in the visual field were specified. The number of these surface cracks identified in the observation field was counted. The interface observed linearly in the observed image was linearly approximated, and the number of surface cracks counted by the length in the observation field of the straight line was divided to obtain the crack number density in the observation field. This operation was performed for 100 visual fields, and the average value of the crack number density in the observed visual field was defined as the crack number density in the steel plate to be measured.

(5) Mechanical properties A tensile test and a limit bending test were performed on the obtained steel sheet.
A) Tensile test JIS No. 5 tensile test was taken from the direction perpendicular to the rolling direction of each steel plate. The test method conformed to JIS Z2241. Yield point (YP), tensile strength (TS), and total elongation (El) were measured.

B) Bending test A test piece having a width of 40 mm and a length of 200 mm was taken from the direction perpendicular to the rolling direction of each steel plate. The test shape and test method conformed to JIS Z2248. The inner radius is 180 times with close contact, 0.5 times the plate thickness, 1.0 times, 1.5 times, 2.0 times, 2.5 times, 3.0 times, 3.5 times, 4.0 times. ° Bending test was conducted, and the minimum value of the inner radius where cracks did not occur was defined as the critical bending radius.

C) Hole expansion test The hole expansion rate (HER%) was measured according to the “JFS T 1001-1996 hole expansion test method” of the Japan Iron and Steel Federation standard, and used as an index of stretch flangeability.

  The test results are shown in Table 4. In Tables 1 to 4, numerical values indicating chemical composition, production conditions, steel structure, and mechanical properties are underlined, indicating that they are outside the scope of the present invention.

  Sample Nos. 1 to 35, which are examples of the present invention, have a TS × El value of 11500 MPa ·% or more, a TS × HER value of 28502 MPa ·% or more, a critical bending radius of 0.5 t to 1.5 t, Excellent in stretchability, stretch flangeability and ductility.

In particular, specimen materials Nos. 16 to 26 containing Bi had an average thickening portion interval of 500 μm or less, a critical bending radius of 0.5 to 1.0 t, and were particularly excellent in bendability.
In addition, before the continuous hot dip galvanizing step, after holding for 5 seconds or more in a temperature range of 750 ° C. or higher, to a temperature range of 250 ° C. or higher and 580 ° C. or lower at an average cooling rate of 2 ° C./second or higher and 200 ° C./second or lower. Nos. 27 to 29 subjected to pre-annealing for cooling had an average super hard phase interval which is an average value of the closest distance of martensite and retained austenite within 10 μm, and a TS × HER value of 55000 MPa ·% or more, It was particularly excellent in stretch flangeability.

  On the other hand, looking at the comparative example, the specimen No. 36 has an average cooling rate of 8 ° C. in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface in the casting process. Since it was too low as / sec, the average interval between the thickening parts was 1050 μm, exceeding the upper limit of the present invention. Therefore, the limit bending radius was 2.5 t, and the bendability was inferior.

  In the test material No. 37, the hot rolling completion temperature in the hot rolling process was too low at 780 ° C., so that the bainite area ratio was as small as 35% in the steel structure of the hot rolled steel sheet. Therefore, the super hard phase average interval exceeded the upper limit of 25 μm and 20 μm, the TS × HER value was 27500 MPa ·%, and the stretch flangeability was inferior.

  Specimen No. 38 had an average cooling rate from completion of hot rolling to winding of 8 ° C./second, which was too low, so that the bainite area ratio was as small as 58% in the steel structure of the hot-rolled steel sheet. Therefore, the average interval between the super hard phases was as large as 28 μm, the TS × HER value was 26850 MPa ·%, and the stretch flangeability was inferior.

  In the test material No. 39, the coiling temperature in the hot rolling process was too high at 600 ° C., and thus the bainite area ratio was as small as 45% in the steel structure of the hot-rolled steel sheet. Therefore, the super hard phase average interval was as large as 30 μm, and the TS × HER value was 26540 MPa ·%, which was inferior in stretch flangeability.

  In the test material No. 40, the coiling temperature in the hot rolling process was too low at 270 ° C., so the bainite area ratio was as small as 40% in the steel structure of the hot-rolled steel sheet. Therefore, the super-hard average interval was as large as 28 μm, the TS × HER value was 25840 MPa ·%, and the stretch flangeability was inferior.

  Specimen No. 41 has an acid concentration (mass%) × acid temperature (° C.) × acid immersion time (seconds) value of 3000 in the pickling process, which is too small, resulting in insufficient pickling, and crack number density. Was as low as 1 piece / mm. Therefore, the limit bending radius was 2.5 t, and the bendability was inferior.

  Specimen No. 42 has an acid concentration (mass%) x acid temperature (° C) x acid soaking time (seconds) value in the pickling process that is too large as 2000500, so that pickling becomes excessive and crack number density Of 1050 / mm. Therefore, the limit bending radius was 2.5 t, and the bendability was inferior.

  Specimen No. 43 had a reduction ratio of 17% in the cold rolling process that was too low, so the reduction in cold rolling was insufficient, and the bainite of the hot-rolled steel sheet was dispersed into dense ferrite and cementite. And the average interval between the super hard phases was as large as 26 μm. Therefore, the TS × HER value was 27210 MPa ·%, and the stretch flangeability was inferior.

  In the test material No. 44, the soaking temperature in the continuous hot dip galvanizing process was too low at 730 ° C., so the ferrite area ratio was as large as 22%. Therefore, the TS value was 760 MPa, and a tensile strength of 780 MPa or more could not be secured.

  In the test material No. 45, the soaking temperature in the continuous hot dip galvanizing process was too high at 1020 ° C., and therefore the ferrite area ratio was as small as 1% in the steel structure. Therefore, the TS × El value was 9860 MPa ·%, and the ductility was inferior.

  In Test Material No. 46, the soaking time in the continuous hot dip galvanizing process was too short, 4 seconds, so the area ratio of retained austenite was 0%. Therefore, the TS × El value was 9450 MPa ·%, which was inferior in ductility.

  In Test Material No. 47, the soaking time in the continuous hot dip galvanizing process was too long, 1010 seconds, so the ferrite area ratio increased to 26%. Therefore, the tensile strength was 775 MPa, and a tensile strength of 780 MPa or more could not be secured.

  In test material No. 48, the average cooling rate of cooling after soaking in the continuous hot dip galvanizing process was too high at 80 ° C./second, so the ferrite area ratio was 0%. Therefore, the TS × El value was 9870 MPa ·%, and the ductility was inferior.

  In Test Material No. 49, the average cooling rate of cooling after soaking in the continuous hot dip galvanizing process was too slow at 1 ° C./second, so the ferrite area ratio increased to 23%. Therefore, the tensile strength was 770 MPa, and a tensile strength of 780 MPa or more could not be secured.

  Specimen No. 50 had a cooling stop temperature after soaking in the continuous hot dip galvanizing process that was too high at 590 ° C., so the bainite area ratio was 50%, the martensite area ratio was 0%, and the ferrite area ratio was 47%. And the steel structure became inappropriate. Therefore, the tensile strength was 720 MPa, and a tensile strength of 780 MPa or more could not be secured.

  Specimen No. 51 had an insufficient bainite area ratio of 58% because the cooling stop temperature for cooling after soaking in the continuous galvanizing process was too low at 280 ° C. Therefore, the TS × HER value was 22020 MPa ·%, and the stretch flange workability was poor.

  In the test material No. 52, the retention time after cooling in the continuous hot dip galvanizing process was too short as 1 second, so the area ratio of retained austenite was 0%. Therefore, the TS × El value was 9830 MPa ·%, and the stretch flangeability was inferior.

  Since the test material No. 53 had an alloying treatment temperature of 720 ° C. which was too high, the area ratio of retained austenite was 0%. Therefore, the TS × El value was 9760 MPa ·%, and the stretch flangeability was inferior.

  In the specimen No. 54, the alloying time was too long as 130 seconds, so that the area ratio of retained austenite was 0%. Therefore, the TS × El value was 9450 MPa ·%, and the stretch flangeability was inferior.

  Specimen No. 55 had a C content of 0.02%, and the chemical composition of the steel was outside the range of the present invention. Therefore, the area ratio of bainite was as low as 30% and the tensile strength was 590 MPa. .

Claims (7)

An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet,
The steel plate
In mass%, C: 0.03% to 0.35%, Si: 0.005% to 2.0%, Mn: 1.0% to 4.0%, P: 0.0004% or more 0.1% or less, S: 0.02% or less, sol.Al: 0.0002% or more and 2.0% or less, N: 0.01% or less, balance Fe and chemical composition consisting of impurities,
The concentrated portion average interval which is the average interval in the direction perpendicular to the rolling direction of the concentrated portion where Mn and / or Si expanded in the rolling direction at a position of a depth of 50 μm from the surface of the steel sheet is 1000 μm or less,
The number density of cracks having a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet is 3 pieces / mm or more and 1000 pieces / mm or less,
The average value of the closest distances of martensite and retained austenite, including area percent, bainite: 60% or more, retained austenite: 1% or more, martensite: 1% or more, and ferrite: 2% or more but less than 20% Having an ultra-hard phase average interval of 20 μm or less,
The alloyed hot-dip galvanized steel sheet has mechanical properties having a tensile strength (TS) of 780 MPa or more.   The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the chemical composition further contains Bi: 0.5 mass% or less, and the concentrated portion average interval is 500 μm or less.   The chemical composition is, by mass%, Ti: 1.0% or less, Nb: 1.0% or less, V: 1.0% or less, W: 1.0% or less, Cr: 1.0% or less, Mo 1 or more types further selected from the group which consists of: 1.0% or less, Cu: 1.0% or less, Ni: 1.0%, and B: 0.01% or less The galvannealed steel sheet according to claim 1 or 2.   The chemical composition is one selected from the group consisting of REM: 0.1% or less, Mg: 0.05% or less, Ca: 0.05% or less, and Zr: 0.05% or less by mass%. Or the alloying hot-dip galvanized steel plate in any one of Claims 1-3 which further contains 2 or more types.   The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 4, wherein the average superhard phase interval is 10 µm or less. It has the following process (A)-(E), The manufacturing method of the galvannealed steel plate in any one of Claims 1-5 characterized by the following:
(A) A casting process in which molten steel is cast under the condition that the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface is 10 ° C./second or more;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 800 ° C or higher, and cooled at an average cooling rate of 10 ° C / second or higher, and 300 ° C or higher. A hot rolling step in which a hot rolled steel sheet is wound up in a temperature range of less than 580 ° C;
(C) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling treatment under conditions satisfying the following formula (1) to form a pickled steel sheet;
(D) a cold rolling step in which the pickled steel plate obtained by the pickling step is subjected to cold rolling at a reduction rate of 20% or more to obtain a cold rolled steel plate; and (E) obtained by the cold rolling step. After holding the cold-rolled steel sheet in a temperature range of 750 ° C. or higher and 1000 ° C. or lower for 5 seconds or longer and 1000 seconds or shorter, an average cooling rate of 2 ° C./second or higher and 70 ° C./second or lower to a temperature range of 300 ° C. or higher and 580 ° C. or lower Cool and hold in this temperature range for 2 seconds or more, then apply hot dip galvanization to obtain a hot dip galvanized steel sheet, and apply an alloying treatment to hold it in a temperature range of 700 ° C. or lower for 120 seconds or less. Continuous galvanizing process.
5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 2000000 (1) It has the following process (a)-(f), The manufacturing method of the galvannealed steel plate in any one of Claims 1-5 characterized by the following:
(A) A casting process in which molten steel is cast under the condition that the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface is 10 ° C./second or more;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 800 ° C or higher, and cooled at an average cooling rate of 10 ° C / second or higher, and 300 ° C or higher. A hot rolling step in which a hot rolled steel sheet is wound up in a temperature range of less than 580 ° C;
(C) a pickling step in which the hot-rolled steel sheet obtained by the hot rolling step is subjected to a pickling treatment under the conditions satisfying the following formula (1) to form a pickled steel plate;
(D) a cold rolling step in which the pickled steel plate obtained by the pickling step is subjected to cold rolling at a reduction rate of 20% or more to obtain a cold rolled steel plate;
(E) The cold-rolled steel sheet obtained by the cold rolling step is held at a temperature range of 750 ° C. or higher for 5 seconds or longer, and then at an average cooling rate of 2 ° C./second or higher and 200 ° C./second or lower at 250 ° C. or higher and 580 ° C. A pre-annealing step in which pre-annealing is performed for cooling to a temperature range of ≦ C ° C .; and (f) the cold-rolled steel sheet obtained by the pre-annealing step is held in a temperature range of 750 ° C. to 1000 ° C. for 5 seconds to 1000 seconds. After that, it is cooled to a temperature range of 300 ° C. or more and 580 ° C. or less at an average cooling rate of 2 ° C./second or more and 70 ° C./second or less and held in this temperature range for 2 seconds or more. A continuous hot-dip galvanizing step in which a steel sheet is subjected to an alloying treatment that is held for 120 seconds or less in a temperature range of 700 ° C. or lower to obtain an alloyed hot-dip galvanized steel sheet.
5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 2000000 (1)