JP2017057460A - High strength alloy galvanized steel, hot rolled steel sheet for the steel sheet and manufacturing method for them - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an alloy galvanized steel sheet having enhanced ultimate deformability.SOLUTION: There is provided high strength alloy galvanized steel sheet having a composition containing, by mass%, C:0.05 to 0.3%, Si:0.05 to 2.5%, Mn:1.50 to 4%, P:0.1% or less, S:0.01% or less, Al:0.01 to 1%, N:0.01% or less and the balance Fe with inevitable impurities, and a steel structure having area percentage of ferrite of 10 to 70%, area percentage of a hard second phase consisting of bainite average hardness of 350 to 550 HV and/or tempered martensite of 30 to 90% and area percentage of retained austenite of 2% or more, standard deviation of line segmentation percentage of the hard second phase on a line drawing in a sheet thickness direction and a vertical direction at each position of depth 3/8 to 1/2t from a steel sheet surface (t:sheet thickness of the steel sheet) of 0.05 or more and (c) ultimate deformation of 0.4 or more in a tensile test in a rolling right angle direction.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a steel plate and a method for manufacturing the same, used for machine structure parts including body structure parts of automobiles, for example. Specifically, the present invention relates to a high-strength galvannealed steel sheet having excellent ductility and hole expandability, a hot-rolled steel sheet for the steel sheet, and a method for producing them.

  Steel sheets used for materials such as automobiles and other transportation machinery and structural members of various industrial machines are required to have mechanical properties excellent in strength, workability, toughness, and the like. In particular, from the viewpoint of reducing the weight of automobiles, the application of high-strength steel sheets has been increasing in recent years.

  Since many automobile parts are manufactured by press molding, high moldability and excellent moldability are required. In particular, high-strength steel sheets applied to members (subframes) and reinforcements (reinforcing members) that are skeleton members of automobiles are required to have not only good ductility but also excellent hole expansibility.

  However, in general, tensile strength and stretch flangeability are in a trade-off relationship, and elongation and hole expandability are significantly reduced as the tensile strength increases. For this reason, it is not easy to achieve both high tensile strength, excellent elongation and hole expansibility.

  For this reason, in high-strength steel sheets, various measures have been taken to improve elongation and hole-expandability, and in particular, steel sheets with improved hole-expandability by tempering martensite have been disclosed. .

  Patent Document 1 proposes a steel sheet having an excellent balance of strength-elongation-hole expansibility, in which the metal structure is controlled to ferrite and tempered martensite by rapid cooling after annealing and subsequent tempering.

  Patent Document 2 discloses a method for producing a steel sheet having excellent hole expanding properties by performing a tempering heat treatment after plating and adjusting the ratio of the tempered martensite hardness and the ferrite hardness to an appropriate range. Has been.

  However, in these methods, the hole expandability is improved, but the strength is lowered by temper softening of martensite, and only a strength of about 920 MPa is obtained at the maximum. Moreover, in order to ensure the strength of 780 MPa or more in TS, hard second phase such as martensite is contained by 30% or more by volume ratio, so that the hard second phase is continuous in a band shape and becomes a stress concentration portion. As a result, the hole expandability deteriorates.

  Therefore, a method for eliminating the band structure and improving the hole expanding property has been reported.

  In Patent Document 3, as shown in the examples, a steel sheet having a martensite fraction of 20% or more is used, and the steel sheet after cold rolling and pickling is once heated to a temperature range of 750 ° C. or more. Disclosed is a steel sheet that is excellent in formability by dispersing Mn concentrated in a grain structure and thinly and finely dispersing a band-like structure.

Patent Document 4 discloses that annealing is performed twice, specifically, the heating temperature Ac _{3 to} 1000 ° C. is held for 3600 seconds during the first annealing, and then cooling is performed at 50 ° C./second. In addition, a steel sheet that has a uniform martensite structure and is annealed for the second time to disperse the major axis direction of ferrite grains isotropically and has excellent stretch flangeability is disclosed.

  On the other hand, in Patent Document 5, in addition to the manufacturing method of Patent Document 4, Mn is diffused by holding at a temperature range of 1200 ° C. to 1300 ° C. for 0.5 to 5 hours before the hot rolling step. Thus, a steel sheet excellent in elongation and stretch flangeability in which the ratio C1 / C2 of the upper limit value C1 and the lower limit value C2 of the Mn concentration in the cross section in the plate thickness direction of the steel sheet is 2.0 or less is disclosed.

  However, since these methods all require a long heating step or a plurality of annealing steps, the productivity is lowered, and the cost of the steel sheet is significantly increased.

  Further, in recent years, as the shape of automobiles becomes more complex, further improvement in hole expansibility has been demanded. Therefore, instead of conventional hole punching after mechanical punching, the method described in Non-Patent Document 1 is a method of removing a damage layer by punching the end face by reaming, or a laser described in Non-Patent Document 2. Practical methods have been put into practical use in which punching is performed to improve hole expansibility so as to eliminate the effect of damage to the punched end face by applying cutting.

  Therefore, it is necessary to evaluate the hole expandability of the material in a situation where there is no such punching damage.

  Hole expansion molding is understood as a fracture phenomenon due to void generation due to damage during punching and void growth-connection due to local deformation during hole expansion. On the other hand, the hole-expanding formability under the condition where there is no punching damage is the generation of voids due to local large deformation-growth-connection, and therefore it is evaluated by the ultimate deformability which is the local deformability of the material. It is effective. That is, in order to improve the hole expansion property when there is no punching damage, it is important to improve the ultimate deformability.

  Also in the conventional knowledge, a method for improving ultimate deformability by tempering heat treatment and band structure control has been reported.

  In Patent Document 6, the segregation of the alloy is eliminated, and the area ratio of the pearlite band having a length of 1 mm or more in the ferrite-pearlite structure is controlled, so that the anisotropy of the ultimate deformability is eliminated and the workability is excellent. A method for manufacturing a hot-rolled steel sheet is disclosed. However, in the method of Patent Document 6, since the band structure is eliminated, it is impossible to achieve both strength of 780 MPa or more and ultimate deformability.

  Patent Document 7 discloses an excellent stretch flangeability limit while having high strength of 780 MPa or more by controlling the nano hardness of ferrite phase, bainite phase, martensite phase and residual γ phase. A method for manufacturing a steel sheet exhibiting deformability is disclosed. However, the method of Patent Document 7 improves the ultimate deformability by eliminating the band structure in the annealing process, so that the strength is only obtained up to 1050 MPa. Furthermore, the method of Patent Document 7 does not include a step of reheating after cooling to a temperature of 300 ° C. or lower, so that hard martensite is generated and stable ultimate deformability cannot be realized. .

JP 2004-052071 A JP 2009-019258 A JP 2002-088447 A JP 2008-097411 A JP 2010-0665307 A International Publication No. 11/090205 JP 2009-001909 A

Journal of JSTP vol.51 no.598 (2010-2011) Iron and Steel, No. 16, 1949-1955, (1985)

  In view of the current state of the prior art, the present invention aims to improve ultimate deformability in an alloyed hot-dip galvanized steel sheet, and to provide a high-strength alloyed hot-dip galvanized steel sheet and a method for producing the same With the goal.

  The inventors of the present invention diligently studied a method for solving the above-mentioned problem, and as a result, by actively utilizing the band-like structure, while ensuring the strength of the steel sheet, softening by tempering martensite, In addition, it has been found that the ultimate deformability in a steel sheet having a high strength of 780 MPa or more can be enhanced by eliminating uneven portions that become stress concentration sites in bainite grains and tempered martensite grains.

  Specifically, unlike ordinary technology, the structure of a hot-rolled steel sheet in which the pearlite grains in the ferrite-pearlite structure are flat and distributed in a band shape, in a two-phase region heating in the hot dip galvanizing process, in a band shape, and The inventors have found that a steel structure in which unevenness of martensite grains has a smooth structure and tempering the structure is less likely to cause stress concentration and can fully utilize the strengthening mechanism by martensite.

  Furthermore, the present inventors control the chemical composition of the steel sheet to a limited range of C, Si, Mn, cold rolling process using a hot-rolled steel sheet having an appropriate pearlite form and two-phase region heat treatment, By using tempering heat treatment, the shape and hardness of the hard second phase are appropriately controlled, and further, by leaving an appropriate amount of austenite, it achieves high strength due to the band structure and harmless formability deterioration, It has been found that a steel sheet having excellent ductility and ultimate deformability can be obtained while having a tensile strength of 780 MPa or more.

  This invention is made | formed based on the said new knowledge, The summary is as follows.

(1) An alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the steel sheet,
(I) By mass%, C: 0.05% to 0.30%, Si: 0.05% to 2.50%, Mn: 1.50% to 4.00%, P: 0.00. 10% or less, S: 0.01% or less, sol. Al: 0.01% or more and 1.00% or less, and N: 0.01% or less, the balance having a chemical composition consisting of Fe and inevitable impurities,
(Ii) (a) The area ratio of the hard secondary phase composed of bainite and / or tempered martensite having an area ratio of ferrite of 10% to 70% and an average hardness of 350HV to 550HV is 30% or more in total. 90% or less, and the area ratio of retained austenite is 2% or more, and (b) along the plate thickness direction at a position of 3 / 8t to 1 / 2t (t: plate thickness of the steel plate) from the steel plate surface. Further, the standard deviation of the line segment ratio of the hard second phase on the line drawn in the direction perpendicular to the plate thickness direction at each position is 0.050 or more, and (c) the ultimate deformability is 0 in the tensile test in the direction perpendicular to the rolling direction. A high-strength galvannealed steel sheet characterized by having a steel structure of 4 or more.

  (2) The chemical composition may be one or two of mass%, Ti: 0.20% or less, Nb: 0.20% or less, and V: 0.20% or less, instead of part of Fe. The high-strength galvannealed steel sheet according to (1) above, which contains seeds or more.

  (3) The chemical composition is mass% in place of part of Fe, Cr: 1.00% or less, Mo: 1.00% or less, Cu: 1.00% or less, and Ni: 1. The high-strength galvannealed steel sheet according to (1) or (2) above, containing one or more of 00% or less.

  (4) The chemical composition is replaced by a part of Fe in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.00. The high-strength galvannealed steel sheet according to any one of (1) to (3) above, containing one or more of 01% or less.

(5) (i) having the chemical composition according to any one of (1) to (4),
(Ii) each made of (a) ferrite and pearlite, (b) at a depth of 3 / 8t to 1 / 2t from the steel plate surface (t: plate thickness of the steel plate), (1b) each along the plate thickness direction The standard deviation of the pearlite line segment on the line drawn in the direction perpendicular to the sheet thickness direction is 0.100 or more, and (2b) the standard deviation of the pearlite cross-sectional length along the sheet thickness direction is Rp / 2 ( (Rp: average value of cross-sectional length in the thickness direction of pearlite grains) or less, and (c) a steel structure in which the ratio of pearlite grains having an aspect ratio of 5 or more to the total pearlite grains is 60% or more. A hot-rolled steel sheet for high-strength galvannealed steel sheets.

(6) A production method for producing the high-strength galvannealed steel sheet according to any one of (1) to (4), wherein the following (A), (B), (C), (D) A process for producing a high-strength galvannealed steel sheet, comprising the steps of (E) and (F).
(A) Pickling the hot-rolled steel sheet for high-strength galvannealed steel sheet as described in (5) above, and then subjecting the hot-rolled steel sheet to cold rolling with a rolling reduction of 40% or more, (B) A cold-rolled steel sheet is heated at an average heating rate of 0.1 ° C./second or more and 100 ° _C./second or less to a maximum temperature of A _C1 point + 50 ° C. or more and less than A _C3 point for 10 seconds or more. Holding step (C) Heating step of cooling from the highest temperature to a temperature range of 500 ° C. to 750 ° C. at an average cooling rate of 3 ° C./second to 100 ° C./second (D) Cold-rolled steel sheet after cooling (E) Plating cooling step of cooling the steel plate after the alloying treatment to 300 ° C. or less at an average cooling rate of 2 ° C./second or more. F) Heat the plated steel sheet to a temperature range of 200 ° C to 600 ° C. Hold 10 minutes or less than 1 second, the tempering step of subjecting the tempering to the steel plate

  (7) A manufacturing method for manufacturing a hot-rolled steel sheet for high-strength galvannealed steel sheets according to (5), comprising the following steps (G) and (H): A method for producing hot-rolled steel sheets for galvannealed steel sheets.

(G) Continuous casting step of continuously casting the molten steel having the chemical composition according to any one of (1) to (4) so that the solidification rate within 80 mm from the slab skin is 50 ° C./min or less. H) Heat treatment process in which the slab is heated to 1100 ° C. or higher and held for 10 minutes or more and less than 2 hours

  According to the present invention, there is provided a high-strength galvannealed steel sheet having high tensile strength of 780 MPa or more and having excellent ductility and hole expandability without performing treatment for eliminating the band structure. be able to.

It is a figure which shows how to obtain | require the line segment rate of the hard 2nd phase which consists of 1 type or 2 types of bainite and tempered martensite in a steel structure.

The high-strength galvannealed steel sheet of the present invention (hereinafter sometimes referred to as “the present galvanized steel sheet”) is an galvannealed steel sheet having an alloyed galvanized layer on the steel sheet surface,
(I) By mass%, C: 0.05% to 0.30%, Si: 0.05% to 2.50%, Mn: 1.50% to 4.00%, P: 0.00. 10% or less, S: 0.01% or less, sol. Al: 0.01 or more and 1.00% or less, and N: 0.01% or less, the balance has a chemical composition consisting of Fe and inevitable impurities,
(Ii) (a) The area ratio of the hard secondary phase composed of bainite and / or tempered martensite having an area ratio of ferrite of 10% to 70% and an average hardness of 350HV to 550HV is 30% or more in total. 90% or less, and the area ratio of retained austenite is 2% or more, and (b) along the plate thickness direction at a position of 3 / 8t to 1 / 2t (t: plate thickness of the steel plate) from the steel plate surface. Further, the standard deviation of the line segment ratio of the hard second phase on the line drawn in the direction perpendicular to the plate thickness direction at each position is 0.050 or more, and (c) the ultimate deformability is 0 in the tensile test in the direction perpendicular to the rolling direction. It has a steel structure that is 4 or more.

The hot-rolled steel sheet for high-strength galvannealed steel sheets of the present invention (hereinafter sometimes referred to as “the hot-rolled steel sheet of the present invention”)
(I) having the chemical composition according to any one of (1) to (4),
(Ii) each made of (a) ferrite and pearlite, (b) at a depth of 3 / 8t to 1 / 2t from the steel plate surface (t: plate thickness of the steel plate), (1b) each along the plate thickness direction The standard deviation of the pearlite line segment on the line drawn in the direction perpendicular to the sheet thickness direction is 0.100 or more, and (2b) the standard deviation of the pearlite cross-sectional length along the sheet thickness direction is Rp / 2 ( (Rp: average value of cross-sectional length in the thickness direction of pearlite grains) or less, and (c) a steel structure in which the ratio of pearlite grains having an aspect ratio of 5 or more to the total pearlite grains is 60% or more. And

The method for producing a high-strength galvannealed steel sheet according to the present invention (hereinafter referred to as “the present invention-plated steel sheet production method”) is a production method for producing the present invention-coated steel sheet, and includes the following (A) and (B): , (C), (D), (E), and (F).
(A) Cold-rolling step of pickling the hot-rolled steel sheet of the present invention and then subjecting the hot-rolled steel sheet to cold rolling with a rolling reduction of 40% or more to obtain a cold-rolled steel sheet (B) Average heating of the cold-rolled steel sheet Heating process at a rate of 0.1 ° C / second or more and 100 ° C / second or less to a maximum temperature of A _C1 point + 50 ° C or more and less than A _C3 point and holding for 10 seconds or more (C) 500 ° C or more from the highest temperature A cooling step of cooling to a temperature range of 750 ° C. or less at an average cooling rate of 3 ° C./second or more and 100 ° C./second or less. (D) Hot-dip galvanizing is applied to the cold-rolled steel sheet after cooling, and then the alloy is heated at the required temperature. (E) Plating cooling step for cooling the alloyed steel sheet to 300 ° C. or less at an average cooling rate of 2 ° C./sec or higher (F) Temperature of the plated steel sheet from 200 ° C. to 600 ° C. Heat to the area and hold for 1 second or more and 10 minutes or less, and tempering the steel sheet Tempering step of performing

The method for producing a hot-rolled steel sheet for high-strength alloyed hot-dip galvanized steel sheet according to the present invention (hereinafter referred to as “the present invention hot-rolled steel sheet production method”) is a production method for producing the hot-rolled steel sheet of the present invention. G) and (H) are provided.
(G) Continuous casting step of continuously casting molten steel having the chemical composition of the plated steel sheet of the present invention so that the solidification rate within 80 mm from the slab skin is 50 ° C./min or less. (H) The slab is made 1100 ° C. or higher. Heat treatment step of heating and holding for 10 minutes or more and less than 2 hours

  The present invention will be described below.

Chemical Composition First, the reasons for limiting the chemical composition of the plated steel sheet of the present invention and the hot rolled steel sheet of the present invention will be described. Hereinafter,% relating to the chemical composition means mass%.

C: 0.05% or more and 0.30% or less C is an important element for enhancing the hardenability of the steel and ensuring the strength. If C is less than 0.05%, it is difficult to ensure a tensile strength of 780 MPa or more, so C is set to 0.05% or more. Preferably it is 0.10% or more. On the other hand, if C exceeds 0.30%, the weldability deteriorates significantly, so C is made 0.30% or less. Preferably it is 0.20% or less.

Si: 0.05% or more and 2.50% or less Si is an important element capable of increasing the tensile strength without degrading the ultimate deformability by solid solution strengthening. If Si is less than 0.05%, the effect of addition is not sufficiently exhibited, so Si is made 0.05% or more. Preferably it is 0.10% or more. On the other hand, if Si exceeds 2.50%, the effect of addition becomes saturated and the occurrence of non-plating in hot dipping becomes a problem, so Si is made 2.50% or less. Preferably it is 2.00% or less.

Mn: 1.50% or more and 4.00% or less Mn is an element necessary for forming the structure in a band shape and improving the strength. When Mn is less than 1.50%, it is difficult to realize a band-like structure, and it is difficult to achieve a strength of 780 MPa or more, so Mn is set to 1.50% or more. 2.0% or more is preferable in that the strength can be increased without adding an expensive alloy element.

  On the other hand, if Mn exceeds 4.00%, the precipitation amount of MnS increases and the low temperature toughness decreases, so Mn is 4.00% or less. From the viewpoint of productivity during hot rolling and cold rolling, 2.50% or less is preferable.

P: 0.10% or less Generally, P is an impurity element, but is also an element having an effect of increasing the tensile strength. If P exceeds 0.10%, the weldability is remarkably reduced, so P is made 0.10% or less. Preferably it is 0.03% or less. P is preferably 0.01% or more from the viewpoint of reliably obtaining the effect of addition.

S: 0.01% or less S is an impurity element. The smaller the S, the more preferable the element. If S exceeds 0.01%, the weldability is remarkably lowered, the precipitation amount of MnS is increased, and the low temperature toughness is lowered. Therefore, S is made 0.01% or less. Preferably it is 0.003% or less, More preferably, it is 0.0015% or less. From the viewpoint of desulfurization cost, S is preferably 0.001% or more.

sol. Al: 0.01% or more and 1.00% or less Al is an element having an action of deoxidizing steel to make the steel plate sound. sol. If the Al content is less than 0.01%, the above effect is not sufficiently exhibited. Al is 0.01% or more. Preferably it is 0.02% or more.

On the other hand, sol. When Al exceeds 1.00%, the weldability is remarkably lowered and the oxide inclusions are increased, so that the surface properties are remarkably deteriorated. Al is 1.00% or less. Preferably it is 0.08% or less. Note that sol. Al means acid-soluble Al that is not an oxide such as Al ₂ O ₃ and is soluble in acid.

N: 0.01% or less N is an impurity element. The smaller the N, the more preferable the element. If N exceeds 0.01%, the weldability is remarkably lowered, so N is made 0.01% or less. Preferably it is 0.006% or less.

Other impurity elements The other impurity elements are not elements intentionally added to the steel sheet, but are elements inevitably mixed from the iron material and / or elements inevitably mixed during the manufacturing process. It is allowed as long as the properties of the plated steel sheet and the hot-rolled steel sheet of the present invention are not impaired.

  In the above chemical composition, the balance is Fe and inevitable impurities.

  In the chemical composition of the present plated steel sheet and the present hot rolled steel sheet, one or more of the following elements may be contained in order to improve the properties of these steel sheets.

Ti: 0.20% or less Nb: 0.20% or less V: 0.20% or less These elements are all elements that contribute to the improvement of steel sheet strength. If any element exceeds 0.20%, the workability deteriorates and hot rolling and cold rolling become difficult. Therefore, any of Ti, Nb, and V is preferably 0.20% or less. . More preferably, any element is 0.15% or less. In terms of obtaining the effect of addition reliably, any element is preferably 0.003% or more.

Cr: 1.00% or less Mo: 1.00% or less Cu: 1.00% or less Ni: 1.00% or less These elements are elements that contribute to the improvement of the steel sheet strength. If any element exceeds 1.00%, the effect of addition is saturated and economically disadvantageous. Therefore, any of Cr, Mo, Cu, and Ni is preferably 1.00% or less. More preferably, any element is 0.07% or less. In order to ensure the effect of addition, any element is preferably 0.005% or more.

Ca: 0.01% or less Mg: 0.01% or less REM: 0.01% or less Zr: 0.01% or less Each of these elements controls the shape of inclusions, and in particular, the inclusions are fine. It is an element that disperses and contributes to improved toughness. If any element exceeds 0.01%, the surface properties are remarkably deteriorated, so any element is preferably 0.01% or less. More preferably, any element is 0.007% or less. In terms of obtaining the effect of addition reliably, any element is preferably 0.0003% or more.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and is at least one of them. The amount of REM means the total amount of at least one of these elements. In the case of a lanthanoid, it is added industrially in the form of misch metal.

Next, the steel structure of the plated steel sheet of the present invention and the steel structure of the hot rolled steel sheet of the present invention will be described.

(Steel structure of the plated steel sheet of the present invention)
The plated steel sheet of the present invention has a total area ratio of (a) hard secondary phase composed of bainite and / or tempered martensite having an area ratio of ferrite of 10% to 70% and an average hardness of 350HV to 550HV. 30% or more and 90% or less, and the area ratio of retained austenite is 2% or more, and (b) the plate thickness at a position of 3 / 8t to 1 / 2t from the steel plate surface (t: plate thickness of the steel plate). The standard deviation of the line segment ratio of the hard second phase on the line drawn in the direction perpendicular to the thickness direction at each position along the direction is 0.050 or more, and (c) ultimate deformation in the tensile test in the direction perpendicular to the rolling direction It has a steel structure with an ability of 0.4 or more.

  Ferrite area in an image taken at 500 times using an optical microscope at the 1/4 position of the width of the steel plate, corroding the thickness cross section in the direction parallel to and perpendicular to the rolling direction by nital etching. Rate and tempered martensite area ratio. In the texture image, the equiaxed region with constant contrast in the crystal grains was made ferrite, and the region containing carbides or shear bands in the crystal grains was made bainite or tempered martensite.

(1) Area ratio of ferrite: 10% or more and 70% or less If the area ratio of ferrite is less than 10%, it becomes difficult to ensure a total elongation of 10% or more. Therefore, the area ratio of ferrite is 10% or more. To do. Preferably it is 20% or more. On the other hand, if the area ratio of ferrite exceeds 70%, the tensile strength decreases, so the area ratio of ferrite is set to 70% or less. Preferably it is 60% or less.

(2) Area ratio of bainite and / or tempered martensite: 30% or more and 90% or less If the total area ratio of bainite and / or martensite is less than 30%, the tensile strength decreases and a tensile strength of 780 MPa or more is obtained. Since it becomes difficult to ensure, the total area ratio of bainite and / or martensite is 30% or more. Preferably it is 40% or more.

  On the other hand, if the total area ratio of bainite and / or martensite exceeds 90%, the ductility decreases, so the total area ratio of bainite and / or martensite is 90% or less. Preferably it is 80% or less.

(3) Area ratio of retained austenite: 2% or more Residual austenite is a structure that contributes to the improvement of elongation by improving the work hardening ability of the steel sheet because it undergoes deformation-induced transformation to martensite with deformation. If the area ratio of retained austenite is less than 2%, it is difficult to ensure sufficient elongation, so the area ratio of retained austenite is 2% or more. Preferably it is 5% or more.

(3) Average hardness of hard second phase: 320 HV or more and 550 HV or less When the hardness of hard second phase (bainite and / or tempered martensite) distributed in a band shape is high, stress localization in crystal grains becomes remarkable. At the same time, the ductility of the hard second phase is lowered, the generation of voids is facilitated, and the ultimate deformability is lowered. If the average hardness of the hard second phase exceeds 550 HV, it becomes difficult to secure an ultimate deformability of 0.4 or more, so the average hardness of the hard second phase is set to 550 HV or less. Preferably it is 450HV or less.

  On the other hand, if the average hardness of the hard second phase is less than 320 HV, it becomes difficult to ensure a strength of 780 MPa or more, so the average hardness of the hard second phase is set to 320 HV or more. Preferably it is 350HV or more.

(4) At a depth of 3 / 8t to 1 / 2t from the steel sheet surface (t: thickness of the steel sheet), the hard line on the line drawn in the direction perpendicular to the thickness direction at each position along the thickness direction. Standard deviation of line segment ratio of two phases: 0.050 or more The ultimate deformability is an index showing local ductility from fracture to the necking of steel sheet and generation and connection of voids in the steel structure. is there. In the tensile deformation in which the steel plate is constricted, the central portion of the steel plate becomes a stress concentration location, and therefore, voids are usually generated around the position of 1/2 t from the steel plate surface. The connection of voids occurs before the steel sheet breaks. However, when the voids are coarsened to a size of 1/8 t or more, the fracture occurs starting from the coarse voids.

  Since the voids generated at the position of 1 / 2t are connected and the voids that are the starting points of the fracture are the hard second phase existing at the position of the depth 1 / 2t to 3 / 8t from the steel plate surface. The position having a dominant influence on the ultimate deformability was defined as a position having a depth of 3 / 8t to 1 / 2t from the steel sheet surface.

  When the standard deviation of the line segment ratio of the hard second phase increases, the steel structure becomes a remarkable band-like structure, the loading capacity increases, and the strength increases. Therefore, in order to ensure the strength of 780 MPa or more, the standard deviation of the line segment ratio of the hard second phase is 0.050 or more. Preferably it is 0.060 or more.

  In addition, each phase in a steel structure, an area rate, and a line segment rate can be measured by the method used in the below-mentioned Example.

(Steel structure of the hot-rolled steel sheet of the present invention)
The hot-rolled steel sheet of the present invention comprises (a) ferrite and pearlite, and (b) at a depth of 3 / 8t to 1 / 2t from the steel sheet surface (t: thickness of the steel sheet), (1b) in the thickness direction The standard deviation of the pearlite line segment on the line drawn in the direction perpendicular to the thickness direction at each position along the line is 0.100 or more, and (2b) the standard deviation of the cross-sectional length of the pearlite along the thickness direction is A steel structure having Rp / 2 (Rp: average value of cross-sectional length in the thickness direction of pearlite grains) or less and (c) the ratio of pearlite grains having an aspect ratio of 5 or more to the total pearlite grains is 60% or more. Have.

(1) Ferrite and pearlite The steel structure of the hot-rolled steel sheet of the present invention consists of ferrite and pearlite. The steel structure is formed by continuously casting molten steel having the chemical composition of the plated steel sheet of the present invention.

(2) At a position of depth 3 / 8t to 1 / 2t from the steel plate surface (t: plate thickness of the steel plate)
(1b) The standard deviation of the pearlite line segment on the line drawn in the direction perpendicular to the thickness direction at each position along the thickness direction: 0.100 or more
(2b) Standard deviation of pearlite cross-sectional length along the plate thickness direction: Rp / 2 (Rp: average value of cross-sectional length of pearlite grains in the plate thickness direction) or less The line segment ratio of the hard second phase on the line drawn in the direction perpendicular to the plate thickness direction at each position along the plate thickness direction at a position of 3 / 8t to 1 / 2t (t: plate thickness of the steel plate). In order to secure a standard deviation of 0.050 or more, in the hot rolled steel sheet of the present invention, at the same position, “(1b) Perlite line on the line drawn in the direction perpendicular to the sheet thickness direction at each position along the sheet thickness direction. Standard deviation of fraction: 0.100 or more, (2b) Standard deviation of cross section length of pearlite along plate thickness direction: Rp / 2 (Rp: average value of cross section length of pearlite grains in thickness direction) or less " It prescribes.

  When the standard deviation of the pearlite line segment is less than 0.100, austenite does not form in a band shape during heating in a two-phase region, soft tempered martensite does not form a band, and a strength of 780 MPa or more. Therefore, the standard deviation of the pearlite line segment ratio is set to 0.100 or more. Preferably it is 0.110 or more.

  A hard second phase having a high aspect ratio increases stress distribution and improves strength compared to an equiaxed structure, while a hard second phase having a constricted region is a hard second phase grain. Causes stress concentration in the interior, and becomes a void generation site.

  In the distribution of the cross-sectional length of pearlite, pearlite grains having a standard deviation exceeding Rp / 2 (Rp: average value of cross-sectional length of pearlite grains in the plate thickness direction) have many constricted regions, which are later melted. In the galvanizing process, it transforms into a constricted hard second phase and tends to reduce the ultimate deformability. Therefore, in the plated steel sheet of the present invention, in order to reduce the constricted hard second phase and improve the ultimate deformability, the standard deviation of the pearlite cross-sectional length along the thickness direction is Rp / 2 (Rp: pearlite). The average value of the cross-sectional length in the grain plate thickness direction) or less.

(3) Ratio of pearlite grains having an aspect ratio of 5 or more to all pearlite grains: 60% or more In the plated steel sheet of the present invention, the aspect ratio of pearlite grains is ensured in order to ensure an ultimate deformability of 0.4 or more in a tensile test in the direction perpendicular to rolling. The ratio is 5 or more, and the ratio of pearlite grains having an aspect ratio of 5 or more to the total pearlite grains is 60% or more. Preferably it is 70% or more.

Mechanical properties The mechanical properties of the plated steel sheet of the present invention will be described. The tensile strength (TS) of the plated steel sheet of the present invention is preferably 780 MPa or more as sufficient strength to contribute to weight reduction of the automobile. For the ductility, it is preferable that the elongation at break El measured by a method specified in JIS Z 2241 is 10% or more using a JIS No. 5 tensile test specimen taken so that the tensile direction is perpendicular to the rolling direction.

The ultimate deformability is usually calculated from the plate width and thickness reduction rate of the fractured portion of the test piece, but in the tensile test of a high strength steel plate of 780 MPa or more, the plate width reduction rate is smaller than the plate thickness reduction rate. Here, the ultimate deformability was evaluated using only the plate thickness reduction rate. The ultimate deformability ε _t is defined by the following equation using the plate thickness t ₀ before deformation and the plate thickness t after fracture.
ε _t = ln (t / t ₀ )

Ultimate deformability in a tensile test in the direction perpendicular to rolling: 0.4 or more In the present invention plated steel sheet, the ultimate deformability is set to 0.4 or more in order to ensure the formability of the automotive steel sheet. Preferably it is 0.5 or more. The ultimate deformability is more preferable in terms of material stability as the variation due to the position of the steel plate is smaller. Specifically, in a test using five tensile test pieces centered on each point when the plate width is divided into six equal parts, the ratio of the minimum value / average value of the ultimate deformability is 0. .9 or more is preferable.

  Next, the manufacturing method of this invention plated steel plate and this invention hot-rolled steel plate is demonstrated.

The manufacturing method of this invention hot-rolled steel plate The manufacturing method of this invention hot-rolled steel plate comprises the following process.
(G) Continuous casting step of continuously casting molten steel having the chemical composition of the plated steel sheet of the present invention so that the solidification rate within 80 mm from the slab skin is 50 ° C./min or less. (H) The slab is made 1100 ° C. or higher. Heat treatment step of heating and holding for 10 minutes or more and less than 2 hours

Continuous casting process ((G) process)
Solidification rate within 80 mm from slab skin: 50 ° C./min or less Normally, when the solidification rate is high, the interval between dendrites becomes fine. When the solidification rate within 80 mm from the slab skin exceeds 50 ° C / min, the interval between the dendrite inside the 80 mm from the slab skin becomes fine, and the alloy segregation region is divided by subsequent hot rolling, and the shape is good Therefore, the solidification rate within 80 mm from the slab skin is 50 ° C./min or less. Preferably it is 45 degrees C / min or less.

Heat treatment process ((H) process)
Heating temperature: 1100 ° C. or higher, holding time: 10 minutes or longer and less than 2 hours, by subjecting the slab to heat treatment at a high temperature for an appropriate time, the minute alloy segregation part existing between the secondary dendrite arms is eliminated, which is more preferable. A band tissue can be formed. When the heating temperature is less than 1100 ° C., the segregation element does not sufficiently diffuse and a preferable band structure cannot be formed. Therefore, the heating temperature is set to 1100 ° C. or higher. Preferably it is 1150 degreeC or more.

  If the holding time is less than 10 minutes, the minute alloy segregation part is not sufficiently eliminated, and the standard deviation of the pearlite line segment ratio is lowered, so the holding time is set to 10 minutes or more. Preferably it is 30 minutes or more. On the other hand, if the holding time is 2 hours or more, the band structure itself is eliminated, and the standard deviation of the pearlite line segment ratio is reduced, so the holding time is set to less than 2 hours. Preferably it is 1.5 hours or less.

Manufacturing method of this invention plated steel plate This invention steel plate manufacturing method is equipped with the process of following (A), (B), (C), (D), (E), and (F).
(A) Cold-rolling step of pickling the hot-rolled steel sheet of the present invention and then subjecting the hot-rolled steel sheet to cold rolling with a rolling reduction of 40% or more to obtain a cold-rolled steel sheet (B) Average heating of the cold-rolled steel sheet Heating process at a rate of 0.1 ° C / second or more and 100 ° C / second or less to a maximum temperature of A _C1 point + 50 ° C or more and less than A _C3 point and holding for 10 seconds or more (C) 500 ° C or more from the highest temperature A cooling step of cooling to a temperature range of 750 ° C. or less at an average cooling rate of 3 ° C./second or more and 100 ° C./second or less. (D) Hot-dip galvanizing is applied to the cold-rolled steel sheet after cooling, and then the alloy is heated at the required temperature. (E) Plating cooling step for cooling the alloyed steel sheet to 300 ° C. or less at an average cooling rate of 2 ° C./sec or higher (F) Temperature of the plated steel sheet from 200 ° C. to 600 ° C. Heat to the area and hold for 1 second or more and 10 minutes or less, and tempering the steel sheet Tempering step of performing

  Hereinafter, each step will be described.

Cold rolling process (process (A))
Reduction ratio: 40% or more After hot-rolling the steel sheet of the present invention, it is subjected to cold rolling with a reduction ratio of 40% or more to obtain a cold-rolled steel sheet. If the rolling reduction is less than 40%, the pearlite part of the steel structure is not sufficiently band-shaped and it is difficult to ensure a strength of 780 MPa or more, so the rolling reduction is set to 40% or more. Preferably it is 50% or more.

Heating process (process (B))
Average heating rate: 0.1 ° C / second or more and 100 ° C / second or less Maximum temperature: A _C1 point + 50 ° C or more and less than A _C3 point Holding time: 10 seconds or more To make the steel structure into a band in the plated steel sheet of the present invention For this, it is necessary to transform the pearlite grains distributed in a band shape in the steel structure of the hot-rolled steel sheet of the present invention into austenite by two-phase heating.

  The average heating rate is set to 0.1 ° C./second or more from the viewpoint of productivity. Preferably, it is 5 ° C./second or more. On the other hand, if the average heating rate exceeds 100 ° C./second, the temperature control becomes difficult and the temperature fluctuation increases, so the average heating rate is set to 100 ° C./second or less. Preferably it is 85 degrees C / sec or less.

If the maximum temperature reached is less than A _C1 point + 50 ° C., austenitization becomes insufficient and a sufficient amount of martensite cannot be secured, so the maximum temperature reached is A _C1 point + 50 ° C. or higher. Preferably, it is A _C1 point + 60 ° C. or higher. On the other hand, if the maximum temperature reached is A _C3 point or higher, martensite is not distributed in a band shape, so the maximum temperature reached is less than A _C3 point. Preferably, it is A _C1 point −10 ° C. or lower.

Here, the A _C1 point and the A _C3 point are temperatures defined by the following equations, respectively.
A _C1 (° C.) = 723-10.7 (% Mn) −16.9 (% Ni)
+29.1 (% Si) +16.9 (% Cr)
A _C3 (° C.) = 910−203√ (% C) −15.2 (% Ni)
+44.7 (% Si) +104 (% V) +31.5 (% Mo)

  If the holding time at the maximum attained temperature is less than 10 seconds, austenitization becomes insufficient, and a sufficient amount of martensite cannot be secured, so the holding time at the highest attained temperature is set to 10 seconds or more. Preferably it is 20 seconds or more.

Cooling step ((C) step)
Cooling temperature range: 500 ° C. or more and 750 ° C. or less Average cooling rate: 3 ° C./second or more and 100 ° C./second or less If the cooling temperature region after holding at the highest temperature is less than 500 ° C., the amount of ferrite produced becomes excessive. Since the strength of 780 MPa or more cannot be ensured, the cooling temperature range is set to 500 ° C. or more. Preferably it is 530 degreeC or more. On the other hand, when the cooling temperature range exceeds 750 ° C., a sufficient amount of ferrite is not generated and elongation decreases, so the cooling temperature range is set to 750 ° C. or less. Preferably it is 720 degrees C or less.

  If the average cooling rate at the time of cooling from the highest temperature is less than 3 ° C / sec, ferrite is excessively generated and the strength of 780 MPa or more cannot be secured, so the average cooling rate is 3 ° C / sec or more. And Preferably, it is 10 ° C./second or more. On the other hand, when the average cooling rate exceeds 100 ° C./second, it becomes difficult to cope with in terms of equipment, so the cooling rate is set to 100 ° C./second or less.

Plating process ((D) process)
The cold-rolled steel sheet after cooling is subjected to hot dip galvanization and then subjected to alloying treatment at a required temperature. Plating and alloying may be performed under normal conditions, and plating conditions and alloying conditions are not limited to specific conditions.

Plating cooling process ((E) process)
After the alloying treatment, the alloying temperature is cooled to 300 ° C. or less at an average cooling rate of 2 ° C./second or more.

Average cooling rate: 2 ° C./second or more Cooling temperature range: 300 ° C. or less When the average cooling rate after alloying treatment is less than 2 ° C./second, the average hardness of the hard second phase is lowered and the strength is 780 MPa or more. Therefore, the average cooling rate after the alloying treatment is 2 ° C./second or more. Preferably, it is 5 ° C./second or more. The upper limit is not particularly limited, but in the case of gas cooling, since it is difficult to achieve a cooling rate exceeding 100 ° C./second, 100 ° C./second is a practical upper limit in terms of equipment.

  When the cooling temperature range exceeds 300 ° C, the average hardness of the hard second phase is lowered by the next tempering, so the cooling temperature range is set to 300 ° C or less. Preferably it is 200 degrees C or less.

Tempering process (process (F))
The steel sheet is heated to a temperature range of 200 ° C. or more and 600 ° C. or less, kept for 1 second or more and 10 minutes or less, and subjected to tempering treatment after cooling after alloying treatment. In this tempering process, martensite is tempered and softened.

Heating temperature: 200 ° C. or more and 600 ° C. or less Holding time: 1 second or more and 10 minutes or less If the heating temperature is less than 200 ° C., the martensite is not tempered sufficiently and the required ultimate deformability cannot be secured. The heating temperature is 200 ° C. or higher. Preferably it is 230 degreeC or more. On the other hand, when the heating temperature exceeds 600 ° C., retained austenite disappears and elongation decreases, so the heating temperature is set to 600 ° C. or less. Preferably it is 570 degrees C or less.

  If the holding time is less than 1 second, the martensite is not sufficiently softened and the ultimate deformability is lowered, so the holding time is 1 second or more. Preferably it is 10 seconds or more. On the other hand, if the holding time exceeds 10 minutes, the tempering effect is saturated and the productivity is lowered, so the holding time is 10 minutes or less. Preferably it is 7 minutes or less.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
The steel having the chemical composition shown in Table 1 was melted, and the conditions shown in Table 2 and Table 3 (continuation of Table 2) were followed by “standard deviation of pearlite line fraction” and “good shape” shown in Table 2 and Table 3. A hot-rolled steel sheet having a steel structure having an area ratio of pearlite grains was produced.

  The hot-rolled steel sheet was pickled, then subjected to cold rolling, and cold-rolled at the “rolling ratio” shown in Tables 2 and 3 to obtain a cold-rolled steel sheet. In the heating step, the cold-rolled steel sheet was heated to the “maximum ultimate temperature” at the “heating rate” shown in Tables 2 and 3, and held at the temperature for the “holding time” shown in Tables 2 and 3. After this holding, cooling was performed to “cooling stop temperature” at the “cooling rate” in the cooling step shown in Tables 2 and 3.

  The cooled cold-rolled steel sheet was heated to a hot dip galvanizing bath temperature of 460 ° C., and then cooled to a “cooling stop temperature” at the “cooling rate” in the plating cooling step shown in Tables 2 and 3. Next, the steel sheet after cooling was subjected to tempering treatment with “tempering temperature” and “tempering time” shown in Tables 2 and 3, and cooled to room temperature.

  The characteristic value of the tempered steel sheet was measured or calculated as follows.

(1) Observation of steel plate cross section The steel structure of the steel plate corrodes the plate thickness cross section in the direction parallel to and perpendicular to the rolling direction at the position of 1/4 of the width of the steel plate by nital etching, and the optical microscope Using, observed at 500 times, the pearlite line fraction of hot-rolled steel sheet and the cross-sectional length distribution of individual pearlite grains, the area ratio of ferrite and the area ratio of bainite and / or tempered martensite of hot-dip galvanized steel sheet The standard deviation of the line segment ratio and the cross-sectional length distribution of bainite and tempered martensite at a position from the depth 3 / 8t to the 1 / 2t position (t: plate thickness of the steel plate) from the steel plate surface was measured by image analysis.

(2) Solidification rate of hot-rolled steel sheet The solidification rate R within 80 mm from the slab skin was corroded with picric acid on the cross-section of the slab cooled to room temperature after continuous casting. It calculated based on the relational expression.
S = 709R ^-0.386

(3) Perlite line fraction along the plate thickness direction of the hot-rolled steel plate Plate thickness direction (depth direction) at a position from 3 / 8t to 1 / 2t (t: plate thickness of the steel plate) from the steel plate surface The line fraction of the hard second phase along was calculated by the following procedure.

  The observation results at the depth of 3 / 8t to 1 / 2t from the steel plate surface obtained by observation of the above-described steel plate cross section were binarized by image processing and divided into two regions, a ferrite portion and a pearlite portion. Since the ferrite portion is a white region and the pearlite is a black region, it can be clearly distinguished.

  Further, the region is divided into pixels of 15t / 4r points at intervals of r / 30 (r: average grain size of ferrite grains) in the plate thickness direction, and the width r / 30 in the divided plate thickness direction and the plate thickness direction. The bainite and martensite line segments in a line segment region having a length of 50r in the vertical direction were obtained at 15t / 4r points, and the standard deviation was calculated.

(4) Section length in the plate thickness direction of pearlite grains Aspect ratio and section length of pearlite grains at a position from a depth of 3/8 t to 1/2 t (t: plate thickness of the steel sheet) from the steel sheet surface, It was determined by the following procedure.

  The observation result of the position from the depth 3 / 8t to 1 / 2t from the steel plate surface obtained by observation of the steel plate cross section is binarized by image processing, and the pearlite region (black region included in the 200 μm × 200 μm region) ), The aspect ratio was obtained by ellipse approximation. For pearlite grains having an aspect ratio of 5 or more, the length in the plate thickness direction (band thickness) was measured at intervals of Rf / 3 (Rf: average grain size of ferrite grains) in the plate width direction. Standard deviation was determined. Furthermore, the area fraction of pearlite grains (perlite grains with good shape) having an aspect ratio of 5 or more and a standard deviation of Rp / 2 or less with respect to all pearlite grains was calculated.

(5) Tensile properties One JIS No. 5 tensile test specimen having a longitudinal direction perpendicular to the rolling direction was taken from a cold-rolled steel sheet at five different positions in the sheet width direction, and the number of trials was n = 5. As [times], tensile properties (tensile strength TS, total elongation El) were measured according to JIS Z 2241.

(6) Ultimate deformability The cross section of the test piece after the tensile test conducted 5 times was observed with an optical microscope, the thickness t of the fractured portion was measured, and the ratio to the original thickness t0. From these, the ultimate deformability (minimum value and average value) was calculated.

(7) Line segment ratio of hard second phase along the plate thickness direction Plate thickness direction (depth direction) at a position from the depth 3 / 8t to 1 / 2t (t: plate thickness of the steel plate) from the steel plate surface ) Along the hard second phase (bainite and / or tempered martensite) was determined by the following procedure.

  The observation result of the position from the depth 3 / 8t to 1 / 2t from the steel sheet surface obtained by observation of the steel sheet cross section is binarized by image processing, and the ferrite part and the bainite and / or the tempered martensite part 2 Divided into two areas. The ferrite portion is a region where the intra-grain contrast is uniform, and the bainite and / or tempered martensite is a region where the intra-grain contrast is not uniform due to the presence of carbides and shear bands.

  The region is divided into pixels of 15t / 4r points at intervals of r / 30 (r: average grain size of ferrite grains) in the plate thickness direction, and the width r / 30 and the plate thickness direction are perpendicular to the divided plate thickness direction. The line segment ratio of bainite and / or martensite in a line segment region having a length of 50r in the direction was obtained for 15t / 4r points, and the standard deviation was calculated.

(8) Residual γ amount An X-ray diffraction pattern was measured at a position of 1/4 of the plate thickness, and a residual γ amount was measured.

  Table 4 summarizes the measurement and calculation results.

  In Tables 1 to 4, the underlined numerical values indicate that they are outside the scope of the present invention.

  In Table 4, the test material No. 2, no. 6, no. 7, no. 8, no. 10, no. 11, no. 12, no. 15, no. 16, no. 18, no. 21, no. 26, no. 27, no. 28, no. 29, no. 30, no. 31, no. 32, no. 33, no. 34, no. 36 and no. 37 is a steel plate of the invention example that satisfies all the conditions of the present invention.

  Specimen No. 1, no. 17 and no. No. 25 is out of the range of the chemical composition of the present invention, and a strength of 780 MPa or more is not obtained.

  Specimen No. 3 and No. No. 35 has low strength because the manufacturing conditions are outside the scope of the present invention and the average hardness of the hard second phase does not reach the scope of the present invention.

  Specimen No. 4, no. 13, no. 19 and No. In No. 20, the manufacturing conditions are outside the scope of the present invention, the required ferrite area ratio and / or the hard second phase area ratio cannot be obtained, and the required strength and elongation cannot be obtained.

  Specimen No. 5, no. 9, no. 38 and no. In No. 39, the manufacturing conditions are out of the range of the present invention, and the standard deviation of the line segment ratio of the hard second phase is low, so that a strength of 780 MPa or more is not obtained.

  Specimen No. 3 and No. In No. 35, the production conditions are out of the scope of the present invention, the average hardness of the hard second phase is low, and the strength of 780 MPa or more is not obtained.

  Specimen No. 22 and no. In No. 24, the manufacturing conditions are outside the scope of the present invention, and the hard second phase is not sufficiently tempered, so that the ultimate deformability of 0.4 or more is not obtained.

  Specimen No. No. 14, because the manufacturing conditions are outside the scope of the present invention, and a sufficient amount of pearlite grains having a good shape is not obtained in the hot-rolled steel sheet, the ultimate deformability of 0.4 or more is not obtained.

  As described above, according to the present invention, high-strength alloyed molten zinc having high tensile strength of 780 MPa or more and having excellent ductility and hole-expanding property without performing treatment for eliminating the band structure. A plated steel sheet can be provided. The high-strength galvannealed steel sheet according to the present invention is suitable for applications in which press forming is performed, such as automobile bodies, especially for applications where ductility and stretch flange forming are indispensable. Therefore, the present invention has high industrial applicability.

Claims (7)

An alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the steel sheet surface,
(I) By mass%, C: 0.05% to 0.30%, Si: 0.05% to 2.50%, Mn: 1.50% to 4.00%, P: 0.00. 10% or less, S: 0.01% or less, sol. Al: 0.01% or more and 1.00% or less, and N: 0.01% or less, the balance having a chemical composition consisting of Fe and inevitable impurities,
(Ii) (a) The area ratio of the hard secondary phase composed of bainite and / or tempered martensite having an area ratio of ferrite of 10% to 70% and an average hardness of 350HV to 550HV is 30% or more in total. 90% or less, and the area ratio of retained austenite is 2% or more, and (b) along the plate thickness direction at a position of 3 / 8t to 1 / 2t (t: plate thickness of the steel plate) from the steel plate surface. Further, the standard deviation of the line segment ratio of the hard second phase on the line drawn in the direction perpendicular to the plate thickness direction at each position is 0.050 or more, and (c) the ultimate deformability is 0 in the tensile test in the direction perpendicular to the rolling direction. A high-strength galvannealed steel sheet characterized by having a steel structure of 4 or more.   Instead of a part of Fe, the chemical composition may be one or more of mass%, Ti: 0.20% or less, Nb: 0.20% or less, and V: 0.20% or less. The high-strength galvannealed steel sheet according to claim 1, which is contained.   The chemical composition is mass% instead of part of Fe, Cr: 1.00% or less, Mo: 1.00% or less, Cu: 1.00% or less, and Ni: 1.00% or less The high-strength galvannealed steel sheet according to claim 1 or 2, characterized by containing one or more of the following.   The chemical composition is replaced by a part of Fe in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less The high-strength alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 3, characterized by containing one or more of the following. (I) having the chemical composition according to any one of claims 1 to 4;
(Ii) each made of (a) ferrite and pearlite, (b) at a depth of 3 / 8t to 1 / 2t from the steel plate surface (t: plate thickness of the steel plate), (1b) each along the plate thickness direction The standard deviation of the pearlite line segment on the line drawn in the direction perpendicular to the sheet thickness direction is 0.100 or more, and (2b) the standard deviation of the pearlite cross-sectional length along the sheet thickness direction is Rp / 2 ( (Rp: average value of cross-sectional length in the thickness direction of pearlite grains) or less, and (c) a steel structure in which the ratio of pearlite grains having an aspect ratio of 5 or more to the total pearlite grains is 60% or more. A hot-rolled steel sheet for high-strength galvannealed steel sheets. It is a manufacturing method which manufactures the high intensity | strength galvannealed steel plate of any one of Claims 1-4, Comprising: (A), (B), (C), (D), (E) below And the manufacturing method of the high intensity | strength galvannealed steel plate characterized by including the process of (F).
(A) The hot-rolled steel sheet for high-strength alloyed hot-dip galvanized steel sheet according to claim 5 is pickled, and then cold-rolled with a rolling reduction of 40% or more to obtain a cold-rolled steel sheet. Cold-rolling step (B) The cold-rolled steel sheet is heated at an average heating rate of 0.1 ° C./second or more and 100 ° _C./second or less to a maximum temperature of A _C1 point + 50 ° C. + A _C3 point and held for 10 seconds or more. (C) Cooling step of cooling at an average cooling rate of 3 ° C./second to 100 ° C./second from the highest temperature to a temperature range of 500 ° C. to 750 ° C. (D) (E) Plating cooling step of cooling the steel plate after alloying to 300 ° C. or less at an average cooling rate of 2 ° C./second or more (F) ) Heat the plated steel sheet to a temperature range of 200 ° C or higher and 600 ° C or lower, Sec and held 10 minutes or less or more, subjected to tempering treatment to the steel plate tempering process A high-strength galvannealed hot-dip galvanized steel sheet for high-strength galvannealed steel sheets according to claim 5, comprising the following steps (G) and (H): Manufacturing method of hot-rolled steel sheet for plated steel sheet.
(G) A continuous casting step of continuously casting the molten steel having the chemical composition according to any one of claims 1 to 4 so that a solidification rate within 80 mm from a slab skin is 50 ° C./min or less (H ) Heat treatment process in which the slab is heated to 1100 ° C or higher and held for 10 minutes or more and less than 2 hours