JP5870861B2 - High-strength hot-dip galvanized steel sheet with excellent fatigue characteristics and ductility and small in-plane anisotropy of ductility and method for producing the same - Google Patents

Description

  The present invention is a high-strength hot-dip galvanized steel sheet excellent in formability suitable mainly for structural members of automobiles, in particular, having a tensile strength TS of 780 MPa or more, further excellent in fatigue characteristics and ductility, and in the in-plane of ductility. The present invention relates to a high-strength hot-dip galvanized steel sheet with small anisotropy and a method for producing the same.

  In recent years, for the purpose of ensuring the safety of passengers in the event of a collision and improving fuel efficiency by reducing the weight of the vehicle body, the tensile strength (TS) is more than 780 MPa, and the application of thin high-strength steel sheets to automotive structural members has been aggressive It is being advanced. In particular, recently, the application of high strength steel sheets with extremely high strength having TS of 980 MPa class and 1180 MPa class has been studied. In addition, automobile structural members are often exposed to corrosive environments. Therefore, high strength hot-dip galvanized steel sheets are mainly applied to automobile structural members from the viewpoint of corrosion resistance.

  On the other hand, since many automobile structural members made of steel plates are formed by press working or the like, the steel sheets for automobile structural members are required to have excellent formability (ductility). However, generally, increasing the strength of a steel sheet causes a decrease in formability. Therefore, the present situation is that a hot-dip galvanized steel sheet having both high strength and excellent formability (ductility) and excellent corrosion resistance is desired.

  In response to such a request, for example, Patent Document 1 relates to a high-strength galvannealed steel sheet, and the composition of the steel sheet in mass% is C: 0.04 to 0.1%, Si: 0.4 to 2.0%, Mn: 1.5 to 3.0%. , B: 0.0005 to 0.005%, P ≦ 0.1%, 4N <Ti ≦ 0.05%, Nb ≦ 0.1%, the balance is composed of Fe and inevitable impurities, and the steel sheet structure is a mixed structure of ferrite and martensite And a technique for making Fe% in the galvannealed layer 5 to 25% has been proposed. According to the technique proposed in Patent Document 1, a high-strength galvannealed steel sheet excellent in formability and plating adhesion with a tensile strength TS of 800 MPa or more is obtained.

  Patent Document 2 relates to a high-strength galvannealed steel sheet, and the composition of the steel sheet in mass% is C: 0.05 to 0.15%, Si: 0.3 to 1.5%, Mn: 1.5 to 2.8%, P: 0.03% or less, S : 0.02% or less, Al: 0.005 to 0.5%, N: 0.0060% or less, balance is Fe and inevitable impurities, and (Mn%) / (C%) ≧ 15 and (Si%) / (C%) There has been proposed a technique in which a composition satisfying ≧ 4 is used, and the steel sheet structure is a structure containing 3 to 20% martensite and retained austenite by volume in ferrite. And according to the technique proposed by patent document 2, it is supposed that the high-strength galvannealed steel plate excellent in the press workability of tensile strength 490-880 MPa will be obtained.

  Patent Document 3 relates to a high-strength plated steel sheet, and the composition of the steel sheet in mass% is C: 0.04 to 0.14%, Si: 0.4 to 2.2%, Mn: 1.2 to 2.4%, P: 0.02% or less, S: 0.01% or less. , Al: 0.002 to 0.5%, Ti: 0.005 to 0.1%, N: 0.006% or less, and (Ti%) / (S%) ≧ 5 is satisfied, and the balance is Fe and inevitable impurities. When the volume ratio of martensite and residual austenite is 6% or more in total and the volume ratio of the hard phase structure of martensite, residual austenite and bainite is α%, α ≦ 50000 × [(Ti %) / 48+ (Nb%) / 93+ (Mo%) / 96+ (V%) / 51}. And according to the technique proposed by patent document 3, it is supposed that the low yield ratio high strength plated steel plate excellent in the hole expansibility of tensile strength 590-880 MPa will be obtained.

  Patent Document 4 relates to a high-strength hot-dip galvanized steel sheet and contains C: 0.001 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.01 to 3%, and Al: 0.001 to 4% in mass%. The composition of the remaining Fe and inevitable impurities, the steel sheet structure consisting of 70 to 97% volumetric ferrite and the second phase consisting of 3 to 30% volumetric austenite and / or martensite, the average of ferrite The structure is such that the grain size is 20 μm or less and the average grain size of the second phase is 10 μm or less, and the plating layer on the surface of the steel sheet contains Al: 0.001 to 0.5% and Mn: 0.001 to 2% by mass, with the balance Zn and It is a plating layer composed of inevitable impurities. Further, the Si content of the steel: X mass%, the Mn content of the steel: Y mass%, the Al content of the steel: Z mass%, the Al content of the plating layer: A mass% A technique has been proposed in which the Mn content of the plating layer: B mass% is 0 ≦ 3− (X + Y / 10 + Z / 3) −12.5 × (A−B). And according to the technique proposed by patent document 4, it is supposed that the high intensity | strength hot-dip galvanized steel plate excellent in the plating adhesiveness and ductility at the time of shaping | molding will be obtained.

  Patent Document 5 relates to a high-strength hot-dip galvanized steel sheet, and the composition of the steel sheet in mass% is C: 0.05 to 0.2%, Si: 0.5 to 2.5%, Mn: 1.5 to 3.0%, P: 0.001 to 0.05%, S: Contains 0.0001 to 0.01%, Al: 0.001 to 0.1%, N: 0.0005 to 0.01%, the balance is composed of Fe and inevitable impurities, and the steel sheet structure contains ferrite and martensite including tempered martensite The area ratio of the ferrite in the entire structure is 30% or more, the area ratio of the martensite is 30 to 50%, and the area ratio of the tempered martensite in the entire martensite is 70% or more. A technology has been proposed. And according to the technique proposed by patent document 5, it is supposed that the high intensity | strength hot-dip galvanized steel plate excellent in the moldability which has the tensile strength of 780 MPa or more will be obtained.

  Patent Document 6 relates to a high-strength hot-dip galvanized steel strip, and the steel strip composition in mass% is C: 0.05 to 0.2%, Si: 0.5 to 2.5%, Mn: 1.5 to 3.0%, P: 0.001 to 0.05%, S: 0.0001 to 0.01%, Al: 0.001 to 0.1%, N: 0.0005 to 0.01%, the balance is composed of Fe and unavoidable impurities, and the steel strip structure contains ferrite and martensite. The area ratio of the ferrite phase occupying the whole is 50% or more, and the area ratio of the martensite is 30 to 50%, and the difference between the maximum tensile strength and the minimum tensile strength in the steel strip is 60 MPa or less. Techniques to do this have been proposed. And according to the technique proposed by patent document 6, it is supposed that the high intensity | strength hot-dip galvanized steel strip excellent in the moldability with the small dispersion | variation in the material in a steel strip will be obtained.

Japanese Patent Laid-Open No. 9-13147 Japanese Patent Application Laid-Open No. 11-296991 JP 2002-69574 A JP 2003-55751 A JP 2010-209392 A JP 2011-32549 A

However, with the techniques proposed in Patent Documents 1 to 6, it is not always possible to obtain a high-strength hot-dip galvanized steel sheet that is excellent in fatigue characteristics and ductility and further has a small ductility in-plane anisotropy.
Since automobile structural members also require durability, the high-strength hot-dip galvanized steel sheet, which is the material for automobile structural members, has excellent fatigue properties in addition to desired strength and formability (ductility). It is also important. When the fatigue characteristics are insufficient, fatigue breakdown due to repeated loads occurs during actual use of the automobile structural member, which is a safety problem. Moreover, when the in-plane anisotropy of the ductility of the high-strength hot-dip galvanized steel sheet is large, it is caused by the in-plane anisotropy of ductility when the steel sheet is pressed into a predetermined automobile structural member shape. A shape defect occurs and becomes a problem. With respect to these problems, the techniques proposed in Patent Documents 1 to 6 do not examine the fatigue characteristics of the high-strength hot-dip galvanized steel sheet and the in-plane anisotropy of ductility.

  In view of such circumstances, the present invention provides a high-strength hot-dip galvanized steel sheet having a TS of 780 MPa or more, excellent fatigue characteristics and ductility, and small ductility in-plane anisotropy, and a method for producing the same. With the goal.

The present inventors have intensively studied to obtain a hot-dip galvanized steel sheet having high strength (tensile strength TS of 780 MPa or more), excellent fatigue characteristics and ductility, and low ductility and in-plane anisotropy. Piled up.
As a result, in order to improve the ductility, by adding a predetermined amount of Si, it is possible to secure an appropriate area ratio of ferrite and enhance the work hardening ability of the ferrite itself, and to increase the temperature in continuous hot dip galvanizing. It has been found that it is effective to reduce the residual unrecrystallized structure as much as possible by appropriately controlling the thermal history of the process. In addition, regarding the strength, the composition of the steel plate used as the substrate is a composition containing a predetermined amount of Si, Mn, Ti, B, and by setting these contents in an appropriate range, a tensile strength TS of 780 MPa or more is obtained. It was found that it is possible to secure.

  Furthermore, in order to improve the fatigue properties, the microstructure of the steel plate to be the substrate is a structure containing ferrite and dispersed martensite, and the composition of the steel plate to be the substrate is a predetermined amount of Si and It has been found that a composition containing B is effective.

  When the microstructure of the steel sheet used as the substrate is a structure containing ferrite and dispersed martensite, the dispersed martensite exhibits the effect of bypassing fatigue cracks. In addition, when a predetermined amount of Si is added to the steel plate to be the substrate, repeated stress hardening of the ferrite, that is, development of a fine dislocation cell structure is promoted when a stress is repeatedly applied to the steel plate. Furthermore, when a predetermined amount of B is added to the steel plate to be the substrate, the effect of suppressing the propagation of fatigue cracks by strengthening the ferrite grain boundaries and refining the ferrite crystal grains can be obtained. As described above, the present inventors have found that the fatigue characteristics of a hot-dip galvanized steel sheet are dramatically improved by optimizing the structure and composition of the steel sheet to be the substrate.

  On the other hand, in order to reduce the in-plane anisotropy of the ductility, the aspect ratio of the ferrite crystal grains (the value obtained by dividing the major axis length of the ferrite crystal grains by the minor axis length) in the microstructure of the steel sheet used as the substrate Is controlled in an appropriate range, so that EL (total elongation) and U.EL (uniform elongation) L direction (rolling direction), D direction (direction inclined 45 ° from the rolling direction), C direction (in the rolling direction) It was found that it is important to reduce the difference in the direction perpendicular to the vertical direction. In addition, it has been found that it is effective to control the thermal history of the temperature rising process in continuous hot dip galvanizing in order to control the aspect ratio of the ferrite crystal grains within an appropriate range. Furthermore, by appropriately controlling the thermal history of the temperature rising process in continuous hot dip galvanizing, it is possible to sufficiently progress recrystallization and suppress non-recrystallized ferrite. I found it effective.

  And by satisfying the above conditions at the same time, the present inventors have a tensile strength TS of 780 MPa or more, high fatigue strength and ductility, and high ductility with low in-plane anisotropy of ductility. It has been found that galvanized steel sheets can be manufactured.

The present invention has been made based on the above findings and has the following features.
[1] A high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet, wherein the steel sheet is in% by mass,
C: 0.05% to 0.20%, Si: 0.8% to 2.0%,
Mn: 2.0% to 3.0%, P: 0.001% to 0.050%,
S: 0.0001% to 0.0100%, Al: 0.001% to 0.300%,
N: 0.0005% to 0.0100%, Ti: 0.005% to 0.050%,
B: contains 0.0003% to 0.0050% or less, wherein the composition and the balance of Fe and unavoidable impurities, and an area ratio of 30% or more of ferrite, as well as have the area ratio of 20% or more of martensite, and the ferrite The sum of the martensite area ratio is 98%, the ferrite has an average crystal grain size of 10 μm or less, and has a structure satisfying the following expression (1). Further, the following expressions (2) and (3) A high-strength hot-dip galvanized steel sheet having a TS of 780 MPa or more , excellent in fatigue characteristics and ductility, and having small in-plane anisotropy of ductility.
Record
| (AR _L + AR _C −2 × AR _D ) / 2 | <0.2 (1)
| (EL _L + EL _C −2 × EL _D ) / 2 | <1.0 (2)
| (U.EL _L + U.EL _C −2 × U.EL _D ) / 2 | <0.6 (3)
However, AR _L ferrite crystal grains of the aspect ratio of the L cross section,
AR _C ferrite grains having an aspect ratio of C cross-section,
AR _D ferrite grains having an aspect ratio of D cross-section,
EL _L is the total elongation in the L direction, EL _C is the total elongation in the C direction,
EL _D is the total elongation in the D direction, U.EL _L is the uniform elongation in the L direction,
U.EL _C is uniform elongation in the C direction, U.EL _D is uniform elongation in the D direction.

[2] In the above [1], in addition to the above composition, Cr: 0.05% to 1.00%, Mo: 0.05% to 0.50%, Ni: 0.05% to 1.00%, Cu: 0.05% in mass% A high-strength hot-dip galvanized steel sheet containing one or more selected from 1.00% or less.

[3] In the above [1] or [2], in addition to the above composition, one or two selected from V: 0.005% to 0.100% and Nb: 0.005% to 0.100% in terms of mass% A high-strength hot-dip galvanized steel sheet characterized by comprising

[4] In any one of the above [1] to [3], in addition to the above composition, Ca: 0.0003% to 0.0050%, Mg: 0.0003% to 0.0050%, REM: 0.0003% to 0.0050 % High-strength hot-dip galvanized steel sheet, containing one or more selected from below.

[5] The high-strength hot-dip galvanized steel sheet according to any one of [1] to [4], wherein the hot-dip galvanized layer is an alloyed galvanized layer.

[6] By mass%,
C: 0.05% to 0.20%, Si: 0.8% to 2.0%,
Mn: 2.0% to 3.0%, P: 0.001% to 0.050%,
S: 0.0001% to 0.0100%, Al: 0.001% to 0.300%,
N: 0.0005% to 0.0100%, Ti: 0.005% to 0.050%,
B: A steel slab containing 0.0003% or more and 0.0050% or less, with the balance being composed of Fe and inevitable impurities, is heated to 1050 ° C or higher, and the finish rolling finish temperature is 800 ° C or higher and 950 ° C or lower. After the hot rolling is completed, cooling is started within 2 s, cooled to 600 ° C. or less at an average cooling rate of 50 ° C./s or more, and wound in a temperature range of 400 ° C. to 550 ° C. A hot-rolled sheet, the hot-rolled sheet, after pickling, cold-rolled at a rolling reduction of 30% or more to form a cold-rolled sheet, and the cold-rolled sheet at an average temperature increase rate of 5 ° C / s or more Primary temperature increase to a temperature range of 580 ° C to 720 ° C, and secondary temperature increase to a temperature range of 750 ° C to 900 ° C at an average temperature increase rate of 2 ° C / s or less following the primary temperature increase After maintaining the temperature in the temperature range of 750 ° C to 900 ° C for 15s to 600s, the average cooling rate in the temperature range of 750 ° C to 550 ° C is set to 5 ° C / s to 450 ° C to 550 ° C Cooled to the temperature range, by applying a galvanizing treatment, an area ratio of 30% or more of ferrite, which has an area ratio of 20% or more of martensite, the sum of the area ratio of the ferrite and the martensite is 98 The ferrite has an average crystal grain size of 10 μm or less, has a structure satisfying the following formula (1), further satisfies the following formulas (2) and (3), and has a TS of 780 MPa or more. A method for producing a high-strength hot-dip galvanized steel sheet having excellent fatigue properties and ductility, and having a small in-plane anisotropy of ductility, characterized by obtaining a hot-dip galvanized steel sheet.
Record
| (AR _L + AR _C −2 × AR _D ) / 2 | <0.2 (1)
| (EL _L + EL _C −2 × EL _D ) / 2 | <1.0 (2)
| (U.EL _L + U.EL _C −2 × U.EL _D ) / 2 | <0.6 (3)
However, AR _L ferrite crystal grains of the aspect ratio of the L cross section,
AR _C ferrite grains having an aspect ratio of C cross-section,
AR _D ferrite grains having an aspect ratio of D cross-section,
EL _L is the total elongation in the L direction, EL _C is the total elongation in the C direction,
EL _D is the total elongation in the D direction, U.EL _L is the uniform elongation in the L direction,
U.EL _C is uniform elongation in the C direction, U.EL _D is uniform elongation in the D direction.

[7] In the above [6], in addition to the composition, Cr: 0.05% to 1.00%, Mo: 0.05% to 0.50%, Ni: 0.05% to 1.00%, Cu: 0.05% A method for producing a high-strength hot-dip galvanized steel sheet, comprising one or more selected from 1.00% or less.

[8] In the above [6] or [7], in addition to the above composition, one or two selected from V: 0.005% to 0.100% and Nb: 0.005% to 0.100% in mass% in addition to the composition A method for producing a high-strength hot-dip galvanized steel sheet, comprising:

[9] In any one of the above [6] to [8], in addition to the composition, Ca: 0.0003% to 0.0050%, Mg: 0.0003% to 0.0050%, REM: 0.0003% to 0.0050 The manufacturing method of the high intensity | strength hot-dip galvanized steel plate characterized by containing 1 type, or 2 or more types chosen from below.

[10] In any one of the above [6] to [9], after performing the hot dip galvanizing treatment, an alloying treatment of hot dip galvanizing is performed in a temperature range of 470 ° C. to 600 ° C. A method for producing high-strength hot-dip galvanized steel sheets.

  According to the present invention, a high-strength hot-dip galvanized steel sheet having high strength (tensile strength TS of 780 MPa or more), excellent fatigue characteristics and ductility, and small ductility in-plane anisotropy can be obtained. By applying the high-strength hot-dip galvanized steel sheet of the present invention to, for example, an automobile structural member, fuel efficiency can be improved by reducing the weight of the vehicle body, and the industrial utility value is very large.

Details of the present invention will be described below.
First, in the high-strength hot-dip galvanized steel sheet according to the present invention, the reason for limiting the component composition of the steel sheet to be the substrate will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.

C: 0.05% or more and 0.20% or less,
C is an important element for strengthening steel, has high solid solution strengthening ability, and is an indispensable element for adjusting the area ratio and hardness when utilizing the structure strengthening by martensite described later. is there. When the C content is less than 0.05%, it becomes difficult to obtain martensite having a required area ratio, and the martensite phase does not harden, so that sufficient strength cannot be obtained. On the other hand, when the C content exceeds 0.20%, the spot weldability is deteriorated and ductility is reduced due to the formation of a segregation layer. Therefore, the C content is 0.05% or more and 0.20% or less, preferably 0.06% or more and 0.15% or less.

Si: 0.8% to 2.0%
Si is an extremely important element in the present invention. The high-strength hot-dip galvanized steel sheet of the present invention is adjusted to a desired composite structure in an annealing process immediately before the hot-dip galvanizing treatment. Here, during annealing, Si has the effect of discharging solid solution C from ferrite to austenite to clean the ferrite and improve ductility. In addition, since solid solution C is discharged from ferrite to austenite to stabilize austenite, martensite is generated even in continuous hot dip galvanizing treatment, which is difficult to rapidly cool, and has the effect of facilitating complex organization. In particular, the stabilization of austenite in the cooling process suppresses the formation of pearlite and bainite, promotes the formation of martensite, and is effective in forming the DP structure and securing the strength.

  Further, Si dissolved in ferrite improves work hardening ability and increases the ductility of the ferrite itself. Furthermore, in the structure containing ferrite solid-strengthened with Si and martensite that is the hard second phase, the ferrite is repeatedly cured under repeated stress conditions, that is, a fine dislocation cell structure develops, and The dispersed martensite exhibits an effect of bypassing fatigue cracks, thereby improving fatigue characteristics.

  In order to obtain the above effects, the Si content needs to be 0.8% or more. On the other hand, when the Si content exceeds 2.0%, an abnormal structure develops and ductility decreases. Therefore, the Si content is 0.8% or more and 2.0% or less, preferably 1.1% or more and 1.7% or less.

Mn: 2.0% to 3.0%
Mn is an effective element for preventing hot embrittlement of steel and ensuring strength. In addition, in the annealing process prior to hot dip galvanizing treatment, hardenability is improved to facilitate complex formation, and at the same time, it suppresses the formation of pearlite and bainite during the cooling process, and transforms from austenite to martensite. To make it easier. In order to obtain such effects, the Mn content needs to be 2.0% or more. On the other hand, when the Mn content exceeds 3.0%, ductility is reduced. Therefore, the Mn content is 2.0% to 3.0%, preferably 2.6% to 2.9%.

P: 0.001% to 0.050%
P is an element that has a solid solution strengthening action and can be added according to a desired strength. In addition, it is an element effective for complex organization in order to promote ferrite transformation. In order to obtain such an effect, the P content needs to be 0.001% or more. On the other hand, if the P content exceeds 0.050%, the weldability is deteriorated, and when alloying the galvanizing, the alloying speed is lowered and the quality of the galvanizing is impaired. Therefore, the P content is 0.001% to 0.050%, preferably 0.005% to 0.030%.

S: 0.0001% or more and 0.0100% or less
S segregates at the grain boundaries and embrittles the steel during hot working, and also exists as a sulfide and reduces local deformability. Therefore, in the present invention, S is a harmful element, and its content needs to be 0.0100% or less. Preferably it is 0.0030% or less, More preferably, it is 0.0020% or less. However, the S content needs to be 0.0001% or more due to production technology constraints.

Al: 0.001% or more and 0.300% or less
Al is an element effective for generating ferrite and improving the balance between strength and ductility. In order to obtain such an effect, the Al content needs to be 0.001% or more. On the other hand, when the Al content exceeds 0.300%, the surface properties are deteriorated. Therefore, the Al content is 0.001% or more and 0.300% or less, preferably 0.005% or more and 0.200% or less.

N: 0.0005% or more and 0.0100% or less
N is an element that deteriorates the aging resistance of steel. Particularly when the N content exceeds 0.0100%, the deterioration of aging resistance becomes significant. Therefore, in the present invention, N is a harmful element, and its content needs to be 0.0100% or less. Preferably it is 0.0070% or less. In the present invention, the smaller the N content, the better. However, the N content needs to be 0.0005% or more due to restrictions on production technology.

Ti: 0.005% to 0.050%
Ti forms precipitates with C, S, and N and contributes effectively to the improvement of strength and toughness. In addition, when B is added, since N is precipitated as TiN, precipitation of BN is suppressed, and the effect of B described below is effectively exhibited. In order to obtain such effects, the Ti content needs to be 0.005% or more. On the other hand, if the Ti content exceeds 0.050%, precipitation strengthening works excessively, leading to a decrease in ductility. Therefore, the Ti content is 0.005% or more and 0.050% or less, preferably 0.010% or more and 0.030% or less.

B: 0.0003% or more and 0.0050% or less
B stabilizes austenite in the cooling process of the annealing process prior to hot dip galvanizing treatment, suppresses the formation of pearlite and bainite from austenite, and the amount of martensite produced in the cooling process after hot dip galvanizing treatment Is an element effective for securing strength. B also strengthens the ferrite grain boundaries and contributes effectively to improving fatigue properties. In order to obtain such effects, the B content needs to be 0.0003% or more. On the other hand, if the B content exceeds 0.0050%, not only the effect is saturated, but also the productivity during hot rolling is lowered. Therefore, the B content is 0.0003% or more and 0.0050% or less, preferably 0.0010% or more and 0.0030% or less.

  In addition to the above component composition, the steel sheet used as the substrate of the high-strength hot-dip galvanized steel sheet of the present invention is further Cr: 0.05% to 1.00%, Mo: 0.05% to 0.50%, Ni: 0.05% to 1.00 % Or less, Cu: 0.05% or more and 1.00% or less, may be contained alone or in combination.

  Cr, Mo, Ni, and Cu not only serve as solid solution strengthening elements, but also stabilize austenite and facilitate complex formation in the cooling process of the annealing process prior to hot dip galvanizing. In order to obtain such effects, it is preferable that the Cr content, the Mo content, the Ni content, and the Cu content are each 0.05% or more. On the other hand, if the Cr content is 1.00%, the Mo content is 0.50%, the Ni content is 1.00% and the Cu content exceeds 1.00%, formability and spot weldability may be deteriorated. Therefore, when adding Cr, Mo, Ni, Cu, Cr content is 0.05% or more and 1.00% or less, Mo content is 0.05% or more and 0.50% or less, Ni content is 0.05% or more and 1.00% or less, Cu content Is preferably 0.05% or more and 1.00% or less.

  In addition to the above-described component composition, the steel sheet that is the substrate of the high-strength hot-dip galvanized steel sheet of the present invention is further at least one selected from V: 0.005% to 0.100% and Nb: 0.005% to 0.100%. You may contain an element individually or in combination.

  V and Nb increase the strength by forming fine precipitates during hot rolling or annealing in hot dip galvanizing. In particular, by adding an appropriate amount of Nb, the austenite phase generated by reverse transformation at the time of annealing in hot dip galvanizing is refined, so the microstructure after annealing is also refined to increase the strength. In order to obtain such effects, the V content and the Nb content are each preferably 0.005% or more. On the other hand, if the V content and the Nb content exceed 0.100%, respectively, the moldability deteriorates. Therefore, when V and Nb are added, the content is preferably 0.005% or more and 0.100% or less, respectively.

  In addition to the above-described component composition, the steel sheet that is the substrate of the high-strength hot-dip galvanized steel sheet of the present invention is further Ca: 0.0003% to 0.0050%, Mg: 0.0003% to 0.0050%, REM: 0.0003% to 0.0050 % Or less may be contained alone or in combination.

  Ca, Mg, and REM are elements used for deoxidation, and are effective elements for spheroidizing the shape of the sulfide and improving the adverse effect of the sulfide on local ductility. In order to obtain this effect, the Ca content, the Mg content, and the REM content are each preferably 0.0003% or more. However, when Ca, Mg, and REM are excessively contained in excess of 0.0050%, inclusions and the like may be increased to cause surface and internal defects. Therefore, when adding Ca, Mg, and REM, it is preferable to make content each 0.0003% or more and 0.0050% or less.

  In the steel sheet that is the substrate of the high-strength hot-dip galvanized steel sheet of the present invention, the components other than the above are Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Ta, O, H, Na, Cl, Zn, Sc, Co, Y, Zr, Hf, and W.

Next, the microstructure of the high-strength hot-dip galvanized steel sheet of the present invention and the reason for limiting the elongation characteristics will be described.
In the high-strength hot-dip galvanized steel sheet of the present invention, the steel sheet as a substrate has a ferrite with an area ratio of 30% or more and martensite with an area ratio of 20% or more, and the ferrite has an average crystal grain size of 10 μm or less, and And has a microstructure that satisfies the following formula (1). In the following equation (1), AR _L ferrite grains having an aspect ratio of L cross-section, AR _C ferrite grains having an aspect ratio of C cross-section, AR _D is a ferrite crystal grain aspect ratio of D cross-section .
| (AR _L + AR _C −2 × AR _D ) / 2 | <0.2 (1)

Ferrite area ratio: 30% or more In the high-strength hot-dip galvanized steel sheet of the present invention, the microstructure of the steel sheet used as the substrate is a composite in which martensite phase is mainly dispersed as a hard phase in soft ferrite rich in ductility. It consists of an organization. In order to ensure sufficient ductility, the area ratio of ferrite must be 30% or more. Preferably it is 34% or more, more preferably 38% or more. However, if the area ratio of the ferrite becomes excessively high, it is difficult to ensure the desired strength, so 80% or less is preferable. The form of ferrite includes acicular ferrite and recovered non-recrystallized ferrite in addition to polygonal ferrite. However, from the viewpoint of securing good ductility, it is preferable to suppress the area ratio of non-recrystallized ferrite to 3% or less.

Martensite area ratio: 20% or more
In order to achieve a TS of 780 MPa or more, the area ratio of martensite including tempered martensite needs to be 20% or more. Preferably it is 24% or more, more preferably 33% or more. However, when the area ratio of martensite becomes excessively high, it becomes difficult to secure desired ductility.

  The area ratio of ferrite and martensite is determined by corroding 3% nital after polishing the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate, and the thickness 1/4 position (in the depth direction from the steel plate surface). 10 positions at a magnification of 2000 times using a scanning electron microscope (SEM), and each structure (ferrite, martensite) using the obtained tissue image. The area ratio is calculated for 10 visual fields and the values are averaged. In the above-mentioned structure image, ferrite has a gray structure (underground structure), and martensite has a white structure.

Average crystal grain size of ferrite: 10 μm or less Refinement of ferrite crystal grains contributes to improvement of fatigue characteristics. Therefore, in the present invention, the average crystal grain size of ferrite is set to 10 μm or less for the purpose of ensuring good fatigue characteristics. Preferably, it is 8 μm or less.
The average crystal grain size of ferrite is determined by corroding 3% nital after polishing the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate, and the plate thickness 1/4 position (in the depth direction from the steel plate surface). 10 positions at a magnification of 2000 using a SEM (scanning electron microscope), and the area of each ferrite crystal grain is obtained using the obtained structure image. The equivalent circle diameter is calculated and the values are averaged.

  In the high-strength hot-dip galvanized steel sheet of the present invention, the microstructure of the steel sheet used as the substrate includes, in addition to ferrite and martensite (including tempered martensite), residual austenite, bainite, tempered bainite, pearlite, cementite, and the like. Even if the carbide is included in an area ratio of 10% or less, the effect of the present invention is not impaired. In order to stably secure a TS of 780 MPa or more, it is preferable that the pearlite area ratio is less than 2%.

| (AR _L + AR _C −2 × AR _D ) / 2 | <0.2 (1)
However, AR _L ferrite grains having an aspect ratio of L cross-section, AR _C ferrite grains having an aspect ratio of C cross-section, AR _D aspect ratio of ferrite grains of D cross-section.

In order to reduce the in-plane anisotropy of ductility, the aspect ratio (AR _L ) of the ferrite crystal grains in the L section, the aspect ratio (AR _C ) of the ferrite crystal grains in the C section, and the aspect ratio of the ferrite crystal grains in the D section It is necessary to reduce the difference between the ratios (AR _D ). Here, when the value on the left side of the above formula (1), that is, the absolute value of (AR _L + AR _C −2 × AR _D ) / 2 is 0.2 or more, the in-plane anisotropy of ductility increases, and the steel sheet is pressed. As a result, various problems such as a defective shape occur when a member having a predetermined shape is formed. Therefore, in the present invention, the absolute value of (AR _L + AR _C −2 × AR _D ) / 2 is specified to be less than 0.2. Preferably it is 0.15 or less.

  In the above, the aspect ratio of the ferrite crystal grains is a value obtained by dividing the major axis length of the ferrite crystal grains by the minor axis length. The L cross section is a vertical cross section parallel to the rolling direction, the D cross section is a vertical cross section parallel to the direction forming 45 ° with the rolling direction, and the C cross section is a vertical cross section parallel to the direction forming 90 ° with the rolling direction. The aspect ratio (major axis length / minor axis length) of the ferrite crystal grains in each cross section is determined by corroding 3% nital after polishing each cross section and using a SEM (scanning electron microscope) at a magnification of 2000 times. Observe 10 fields of view, measure the aspect ratio of 10 fields of view, and average those values.

In addition, the high-strength hot-dip galvanized steel sheet of the present invention has elongation characteristics that satisfy the following expressions (2) and (3). In the following equations (2) and (3), EL _L is the total elongation in the L direction, EL _C is the total elongation in the C direction, EL _D is the total elongation in the D direction, and U.EL _L is the total elongation in the L direction. Uniform elongation, U.EL _C is uniform elongation in the C direction, and U.EL _D is uniform elongation in the D direction.
| (EL _L + EL _C −2 × EL _D ) / 2 | <1.0 (2)
| (U.EL _L + U.EL _C −2 × U.EL _D ) / 2 | <0.6 (3)

The above formulas (2) and (3) are both formulas representing an index of ductile in-plane anisotropy. In order to reduce the in-plane anisotropy of ductility, it is important to reduce the difference between EL (total elongation) and U.EL (uniform elongation) in the L direction, D direction, and C direction. Therefore, the absolute value of (EL _L + EL _C −2 × EL _D ) / 2 (value on the left side of equation (2)) and the absolute value of (U.EL _L + U.EL _C −2 × U.EL _D ) / 2 (The value on the left side of equation (3)) is preferably smaller. In the present invention, the absolute value of (EL _L + EL _C −2 × EL _D ) / 2 is less than 1.0, and (U.EL _L + U.EL _C −2 The absolute value of × U.EL _D ) / 2 is less than 0.6.

  In addition, the composition, structure | tissue, and thickness (attachment amount) of the hot-dip galvanized layer of the high-strength hot-dip galvanized steel sheet of the present invention are not particularly limited. In the present invention, the hot dip galvanized layer may be an alloyed galvanized layer.

Next, the manufacturing method of the high intensity | strength hot-dip galvanized steel plate of this invention is demonstrated.
The high-strength hot-dip galvanized steel sheet of the present invention heats a steel slab having the above component composition to 1050 ° C. or higher, performs hot rolling in a temperature range of 800 ° C. or higher and 950 ° C. or lower, and performs hot rolling. After completion of cold rolling, cooling (forced cooling) is started within 2 s, cooling to 600 ° C. or less at an average cooling rate of 50 ° C./s or more, and winding hot rolled sheets in a temperature range of 400 ° C. to 550 ° C. The hot-rolled sheet is pickled and then cold-rolled at a rolling reduction of 30% or more to obtain a cold-rolled sheet, and the cold-rolled sheet is subjected to an average temperature increase rate of 5 ° C / s or more at 580 ° C or more and 720 ° C. A primary temperature rise that rises to a temperature range of ℃ or less, and a secondary temperature rise that rises to a temperature range of 750 ° C or more and 900 ° C or less at an average temperature rise rate of 2 ° C / s or less following the primary temperature rise. And hold for 15 s to 600 s in the temperature range of 750 ° C to 900 ° C, then set the average cooling rate in the temperature range of 750 ° C to 550 ° C to 5 ° C / s to 450 ° C to 550 ° C It can be manufactured by cooling and hot dip galvanizing.

Steel slab heating temperature: 1050 ° C or higher Precipitates produced during casting exist in the steel slab, and these precipitates need to be remelted in the heating stage of the steel slab. This is because when the precipitates remain in the steel slab after heating, these precipitates become coarse precipitates in the finally obtained steel sheet and do not contribute to the steel sheet strength. In the present invention, it is necessary to re-dissolve Ti and Nb-based precipitates deposited during casting by heating the steel slab, and heating at 1050 ° C. or higher confirms the contribution to strength. It is also advantageous to heat the steel slab to 1050 ° C or higher from the viewpoint of scaling off defects such as bubbles and segregation on the surface of the slab, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface. . Therefore, the heating temperature of the slab is set to 1050 ° C. or higher, preferably 1150 ° C. or higher.

However, if the heating temperature of the steel slab becomes excessively high, the austenite crystal grains are coarsened, and as a result, the final structure is coarsened and the fatigue characteristics are deteriorated. The temperature is preferably 1300 ° C. or lower.
The steel slab is preferably produced by a continuous casting method from the viewpoint of preventing macro segregation, but can also be produced by an ingot-making method or the like.

Finishing rolling finish temperature of hot rolling: 800 ° C. or higher and 950 ° C. or lower The heated steel slab is hot rolled by rough rolling and finish rolling to form a hot rolled sheet. At this time, if the finish rolling finish temperature exceeds 950 ° C, the amount of oxide (scale) generated increases rapidly, the interface between the iron and the oxide becomes rough, and the surface quality after pickling and cold rolling deteriorates. Tend to. Further, removal of the hot-rolled scale becomes difficult as the amount of oxide (scale) generated increases, and if some of the hot-rolled scale remains after pickling, the fatigue characteristics and spot weldability are adversely affected. Furthermore, when the finish rolling finish temperature exceeds 950 ° C., the crystal grain size becomes excessively large, and the surface of the pressed product may be roughened during processing.

  On the other hand, when the finish rolling finish temperature is less than 800 ° C., the rolling load increases and the rolling load increases. Also, if the finish rolling finish temperature is less than 800 ° C, the reduction ratio of austenite in the non-recrystallized state becomes high, an abnormal texture develops, the in-plane anisotropy in the final product becomes remarkable, and the material uniformity Not only is impaired, but ductility itself is also reduced. Accordingly, the finish rolling end temperature is set to 800 ° C. or higher and 950 ° C. or lower, preferably 850 ° C. or higher and 930 ° C. or lower.

  The steel slab is made into a sheet bar by rough rolling under normal conditions. However, if the heating temperature of the steel slab is lowered, a bar heater before finish rolling is used from the viewpoint of preventing problems during hot rolling. It is preferable to heat the sheet bar by using, for example. In addition, when the steel slab is subjected to rough rolling, if the steel slab after casting is in a desired temperature range (1050 ° C. or more), the steel slab is heated directly or after being heated for a short time without heating. Also good.

Cooling condition after hot rolling is completed: Cooling (forced cooling) is started within 2 s and cooled to 600 ° C. or less at an average cooling rate of 50 ° C./s or more. In the present invention, the microstructure of the hot-rolled sheet is ferrite and bainite. It is preferable that the organization is mainly composed of. Since bainite contains a lot of dislocations, it becomes the starting point of austenite reverse transformation in the temperature rising process of the annealing process prior to the hot dip galvanizing process, so that the steel sheet contains martensite in a predetermined ratio after the annealing and hot dip galvanizing process. It becomes effective in the above.

  When the time exceeding 2 seconds elapses before the start of cooling (forced cooling) after the hot rolling is finished, ferrite is likely to be generated unevenly on the run-out table, and the ferrite and bainite suitable in the present invention are mainly composed. A uniform microstructure of the hot-rolled sheet cannot be obtained, and it becomes difficult to produce a hot-dip galvanized steel sheet having a small ductility in-plane anisotropy. In addition, when the average cooling rate is less than 50 ° C./s, or when the average cooling rate is 50 ° C./s or higher and the average cooling rate is not cooled to 600 ° C. or lower, the same problem as described above occurs. Therefore, after completion of hot rolling, cooling (forced cooling) is started within 2 s, and cooling is performed to 600 ° C. or less at an average cooling rate of 50 ° C./s or more. In addition, from the viewpoint of avoiding the shape defect that hinders the operation and the meandering resulting therefrom, the average cooling rate is preferably 500 ° C./s or less.

Winding temperature: 400 ° C. or more and 550 ° C. or less In the present invention, after forced cooling to 600 ° C. or less at an average cooling rate of 50 ° C./s or more, the forced cooling is stopped and in a temperature range of 400 ° C. or more and 550 ° C. or less. Winding and hot rolled sheet. When the winding temperature exceeds 550 ° C., pearlite is generated during winding and holding, and a problem arises in that the plate thickness varies with the coil length during subsequent cold rolling. When such a defect occurs, the plateability is impaired, and further, the in-plane anisotropy of the ductility of the final product increases. On the other hand, when the coiling temperature is less than 400 ° C., the hot rolled sheet strength increases, the rolling load in cold rolling increases, and the productivity decreases. Therefore, the coiling temperature is 400 ° C. or higher and 550 ° C. or lower, preferably 400 ° C. or higher and 500 ° C. or lower.

  In addition, when performing hot rolling on the rough rolled plate (sheet bar) obtained by rough rolling at the time of hot rolling, the rough rolled plates may be joined to each other for continuous rolling. Moreover, you may wind up a rough rolling board once.

  The hot-rolled sheet thus manufactured is pickled. Pickling is important for ensuring good plating quality of the final product hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet because it can remove oxide on the surface of the hot-rolled sheet. In the present invention, pickling may be performed once, or pickling may be performed in a plurality of times. The hot-rolled sheet after pickling is cold-rolled to form a cold-rolled sheet.

Rolling ratio during cold rolling: 30% or more When the rolling reduction ratio during cold rolling is less than 30%, the grain boundary and the dislocation per unit volume that become the core of reverse transformation to austenite during subsequent annealing The total number decreases, and it becomes difficult to make the structure of the steel sheet to be the substrate into the desired microstructure as described above. In addition, non-uniformity occurs in the microstructure and ductility decreases. Therefore, the rolling reduction during cold rolling is 30% or more, preferably 40% or more. However, if the rolling reduction during cold rolling becomes excessively high, there is a concern about the in-plane thickness accuracy of the resulting cold-rolled sheet, so 80% or less is preferable.

  The number of rolling passes at the time of cold rolling and the reduction rate of each pass are not particularly specified, and the effect of the present invention is exhibited as long as the total reduction rate at the time of cold rolling is 30% or more. The cold-rolled sheet obtained by cold rolling is subjected to an annealing process and a hot-dip galvanizing process under the following conditions to form a hot-dip galvanized steel sheet.

Average temperature increase rate for primary temperature increase (average temperature increase rate up to a temperature range of 580 ° C or more and 720 ° C or less): 5 ° C / s or more The temperature increase condition during annealing is one of the important manufacturing factors in the present invention. It is. In the annealing process, if the average heating rate to a temperature range of 580 ° C to 720 ° C is less than 5 ° C / s, the recovery of the rolled structure proceeds and ferrite grains with a large aspect ratio are generated. As a result, the in-plane anisotropy of ductility increases. In addition, dislocations are reduced by the progress of recovery, the progress of recrystallization at the subsequent secondary temperature rise is inhibited, and the in-plane anisotropy of ductility is increased.

For the above reasons, in the present invention, it is necessary to set the average rate of temperature rise to a temperature range of 580 ° C. or more and 720 ° C. or less during annealing treatment to 5 ° C./s or more. Preferably it is 7 degrees C / s or more.
However, if the average rate of temperature increase to a temperature range of 580 ° C. or more and 720 ° C. or less exceeds 80 ° C./s, there is a concern that in-plane structure unevenness may occur due to in-plane temperature unevenness in the steel sheet. It is preferable to set it to ℃ / s or less.

Average temperature increase rate of secondary temperature increase (average temperature increase rate when the temperature is raised to a temperature range of 750 ° C. or more and 900 ° C. or less): 2 ° C./s or less The temperature increase conditions during annealing are important in the present invention. One of the manufacturing factors. In the present invention, the temperature is increased from 580 ° C. to 720 ° C. at an average temperature increase rate of 5 ° C./s or more (primary temperature increase), and then further heated to an annealing temperature of 750 ° C. to 900 ° C. The temperature is raised to (secondary temperature rise). Here, if the average rate of temperature rise during further temperature rise (secondary temperature rise) exceeds 2 ° C / s, the recrystallization does not progress sufficiently, not only the ductility decreases but also the ductility surface. The internal anisotropy also increases. Therefore, in the present invention, the average rate of temperature increase when the temperature is further increased (secondary temperature increase) is set to 2 ° C./s or less.

  In the present invention, by controlling the heating rate during the annealing process as described above, the recrystallization is sufficiently advanced, the non-recrystallized ferrite is suppressed and the aspect ratio of the ferrite crystal grains is optimized. The difference in the L direction, D direction, and C direction of (total elongation) and U.EL (uniform elongation) is reduced. That is, in the present invention, controlling the heating rate during the annealing treatment as described above is extremely important in producing a high-strength hot-dip galvanized steel sheet that satisfies the above-mentioned formulas (1) to (3).

Annealing temperature: 750 ° C. or more and 900 ° C. or less If the annealing temperature is less than 750 ° C., sufficient austenite is not generated during annealing, so that a predetermined amount of martensite cannot be obtained in the subsequent cooling process, making it difficult to ensure strength. On the other hand, if the annealing temperature exceeds 900 ° C., ferrite is not sufficiently generated and ductility is lowered. Accordingly, the annealing temperature is set to 750 ° C. or more and 900 ° C. or less. Preferably they are 760 degreeC or more and 870 degrees C or less.

Holding time at annealing temperature: 15 s to 600 s If the holding time at the annealing temperature (residence time in the temperature range of 750 ° C. to 900 ° C.) is less than 15 s, the progress of recrystallization is insufficient, Not only is the ductility lowered, but the in-plane anisotropy of the ductility is also increased. On the other hand, when the holding time at the annealing temperature exceeds 600 s, the ferrite crystal grains become coarse and it is difficult to ensure desired fatigue characteristics. Moreover, productivity is also inhibited. Therefore, the holding time at the annealing temperature is 15 s or more and 600 s or less. Preferably, it is 30 seconds or more and 550 seconds or less.

After annealing, average cooling rate in the temperature range of 750 ° C or lower and 550 ° C or higher: 5 ° C / s or higher After annealing, if the average cooling rate in the temperature range of 750 ° C or lower and 550 ° C or higher is lower than 5 ° C / s, cooling process The amount of pearlite generated therein increases, making it difficult to secure the desired martensite area ratio, and ensuring strength is difficult. Therefore, in the present invention, after holding for a predetermined time (15 s to 600 s) in the temperature range of 750 ° C. to 900 ° C., the average cooling rate in the temperature range of at least 750 ° C. to 550 ° C. becomes 5 ° C./s or more. To cool. Preferably it is 7 degrees C / s or more. However, when the average cooling rate in the temperature range of 750 ° C. or lower and 550 ° C. or higher becomes excessively high, there is a concern about the deterioration of the shape of the steel sheet.

Cooling stop temperature: 450 ° C. or more and 550 ° C. or less When the cooling stop temperature exceeds 550 ° C., the progress of ferrite transformation is not sufficient, and it becomes difficult to secure a desired ferrite area ratio, resulting in a decrease in ductility. On the other hand, when the cooling stop temperature is less than 450 ° C., the bainite transformation proceeds and it becomes difficult to ensure good uniform ductility. In addition, even if it hold | maintains in the said temperature range after cooling stop, the effect of this invention can be acquired.

Hot-dip galvanizing treatment In the present invention, hot-dip galvanizing treatment conditions are not particularly limited. Further, after the hot dip galvanizing treatment, the alloying treatment may be performed.

Alloying temperature: 470 ° C. or higher and 600 ° C. or lower When alloying is performed, the alloying temperature is preferably 470 ° C. or higher and 600 ° C. or lower. If the alloying temperature is less than 470 ° C., alloying does not proceed sufficiently, and there is a possibility that the sacrificial anticorrosive action and the sliding property are lowered. On the other hand, when the alloying temperature exceeds 600 ° C., a part of untransformed austenite at the time of alloying treatment is decomposed to generate carbides, and it may be difficult to ensure strength. Further, there is a concern that the alloying proceeds excessively and the powdering resistance is lowered.

  For the purpose of adjusting the surface roughness, etc., skin pass rolling may be applied to the hot dip galvanized steel sheet after the hot dip galvanizing process or after the alloying process. The rolling reduction of the skin pass rolling is preferably in the range of 0.1% to 1.0%. If the rolling reduction of skin pass rolling is less than 0.1%, the rolling effect is small and control is difficult. On the other hand, when the rolling reduction of the skin pass rolling exceeds 1.0%, the productivity is remarkably lowered. The skin pass may be performed inline or offline. In addition, a skin pass with a desired reduction rate may be performed at once, or may be performed in several steps.

  The conditions for other production methods are not particularly limited, but from the viewpoint of productivity, the series of treatments such as annealing, hot dip galvanizing, and alloying treatment are preferably performed in a continuous hot dip galvanizing line (CGL). For hot dip galvanizing, it is preferable to use a galvanizing bath having an Al content of 0.10% or more and 0.20% or less. After plating, wiping is possible to adjust the basis weight of plating.

  Steel having the composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in a converter, and a steel slab was formed by a continuous casting method. The obtained steel slab is hot-rolled under the conditions shown in Table 2 to obtain hot-rolled sheets, and then the obtained hot-rolled sheets are pickled and cold-rolled under the conditions shown in Table 2 to be cooled. It was a sheet. The obtained cold-rolled sheet was passed through a continuous hot dip galvanizing line (CGL), annealed under the conditions shown in Table 3, and subjected to hot dip galvanizing treatment to obtain a hot dip galvanized steel sheet (GI steel sheet). Further, some of the hot dip galvanized steel sheets were subjected to an alloying process at the alloying temperatures shown in Table 3 after the hot dip galvanizing process to obtain alloyed hot dip galvanized steel sheets (GA steel sheets).

When manufacturing a GI steel plate, a zinc bath containing Al: 0.19% by mass was used as a hot dip galvanizing bath. On the other hand, when manufacturing a GA steel plate, a zinc bath containing Al: 0.14% by mass was used as a hot dip galvanizing bath. In either case of the GI steel sheet and the GA steel sheet, the bath temperature of the hot dip galvanizing bath was 465 ° C., and the amount of plating was 45 g / m ² per side (double-side plating). Further, the Fe concentration in the alloyed galvanized layer of the GA steel sheet was set to 9% by mass or more and 12% by mass or less.

  In the hot rolling conditions described in Table 2, the average cooling rate is the average cooling rate from the finish rolling end temperature to the forced cooling stop temperature. In the annealing conditions described in Table 3, the primary heating rate is the average heating rate in the temperature range from room temperature to the primary attainment temperature, and the secondary heating rate is the average in the temperature range from the primary attainment temperature to the annealing temperature. The rate of temperature increase. Moreover, in the annealing conditions described in Table 3, the annealing time is a residence time in a temperature range of 750 ° C. to 900 ° C. However, the annealing time in the steel plates No. 17 and 18 is the holding time at each annealing temperature. Furthermore, in the annealing conditions described in Table 3, the average cooling rate is an average cooling rate in a temperature range of 750 ° C. or lower and 550 ° C. or higher. However, the average cooling rate in the steel plate No. 22 is an average cooling rate in a temperature range of 750 ° C. or lower and 700 ° C. or higher.

  The obtained hot-dip galvanized steel sheet (GI steel sheet) and alloyed hot-dip galvanized steel sheet (GA steel sheet) were subjected to structure observation, tensile test and fatigue test according to the following methods.

<Tissue observation>
After polishing the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate (GI steel plate or GA steel plate), it corrodes with 3% nital, and the plate thickness is 1/4 position (the plate thickness is 1 in the depth direction from the steel plate surface). / 10 position) using a SEM (scanning electron microscope) and observing 10 visual fields at a magnification of 2000 times, and the obtained tissue images were obtained from each tissue (ferrite) using Image-Pro of Media Cybernetics. , Martensite) area ratio was calculated for 10 visual fields, and these values were averaged to obtain the ferrite area ratio and martensite area ratio. In addition, for the tissue image obtained in the same manner as described above, the area of each ferrite crystal grain was calculated using Image-Pro of Media Cybernetics, the equivalent circle diameter was calculated, and these values were averaged to obtain the ferrite image. The average crystal grain size was determined.
Further, after polishing the L cross section of a steel plate (GI steel plate or GA steel plate), it corrodes with 3% nital, and observes 10 fields of view at a magnification of 2000 using a SEM (scanning electron microscope). The aspect ratio of 10 fields of view was measured using Image-Pro of Media Cybernetics, and the values were averaged to determine the aspect ratio (major axis length / minor axis length) of the ferrite crystal grains in the L section. Similarly, the aspect ratio (major axis length / minor axis length) of the ferrite crystal grains of the C cross section and the D cross section was also obtained.

<Tensile test>
From a steel plate (GI steel plate or GA steel plate), the tensile direction is parallel to the rolling direction of the steel plate (L direction), the tensile direction is 45 ° direction (D direction) with respect to the rolling direction of the steel plate, and the tensile direction is rolling of the steel plate. JIS No. 5 test specimens were sampled so as to be in three directions of 90 ° direction (C direction) with respect to the direction. Using these test pieces, a tensile test was performed in accordance with JIS Z 2241 (1998), and TS (tensile strength), EL (total elongation), and U.EL (uniform elongation) were measured. In the present invention, the TS780MPa class has EL ≧ 24% and U.EL ≧ 13%, the TS980MPa class has EL ≧ 17% and U.EL ≧ 9%, and the TS1180MPa class has EL ≧ 13% and U.EL ≧ 7% The mechanical properties were judged to be good.

<Fatigue test>
In accordance with JIS Z 2275 (1978), plane bending fatigue test specimens were collected from steel sheets (GI steel sheets or GA steel sheets) along the C direction, and the conditions of double swing (stress ratio -1) and frequency 20 Hz were obtained. Then, a double swing plane bending fatigue test was conducted. The stress rupture up to 10 ⁷ cycles in both swing plane bending fatigue test was observed was measured and the stress and fatigue limit strength. In the present invention, it was determined that the fatigue characteristics were good when the TS780 MPa class had fatigue limit strength ≧ 320 MPa, the TS980 MPa class had fatigue limit strength ≧ 400 MPa, and the TS1180 MPa class had fatigue limit strength ≧ 480 MPa. Further, in any of the TS780MPa class, the 980MPa class, and the 1180MPa class, it was determined that the durability ratio was good when the durability ratio ≧ 0.40.
The results obtained as described above are shown in Tables 4-6.

  All the steel plates (GI steel plates or GA steel plates) of the present invention have TS of 780 MPa or more, excellent fatigue characteristics and ductility, and small in-plane anisotropy of ductility. On the other hand, in the comparative example, one or more of strength, fatigue characteristics, and ductility is inferior, and the in-plane anisotropy of ductility is large.

Claims (10)

A high-strength hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet, wherein the steel sheet is in mass%,
C: 0.05% to 0.20%, Si: 0.8% to 2.0%,
Mn: 2.0% to 3.0%, P: 0.001% to 0.050%,
S: 0.0001% to 0.0100%, Al: 0.001% to 0.300%,
N: 0.0005% to 0.0100%, Ti: 0.005% to 0.050%,
B: contains 0.0003% to 0.0050% or less, wherein the composition and the balance of Fe and unavoidable impurities, and an area ratio of 30% or more of ferrite, as well as have the area ratio of 20% or more of martensite, and the ferrite The sum of the martensite area ratio is 98%, the ferrite has an average crystal grain size of 10 μm or less, and has a structure satisfying the following expression (1). Further, the following expressions (2) and (3) A high-strength hot-dip galvanized steel sheet having a TS of 780 MPa or more , excellent in fatigue characteristics and ductility, and having small in-plane anisotropy of ductility.
Record
| (AR _L + AR _C −2 × AR _D ) / 2 | <0.2 (1)
| (EL _L + EL _C −2 × EL _D ) / 2 | <1.0 (2)
| (U.EL _L + U.EL _C −2 × U.EL _D ) / 2 | <0.6 (3)
However, AR _L ferrite crystal grains of the aspect ratio of the L cross section,
AR _C ferrite grains having an aspect ratio of C cross-section,
AR _D ferrite grains having an aspect ratio of D cross-section,
EL _L is the total elongation in the L direction, EL _C is the total elongation in the C direction,
EL _D is the total elongation in the D direction, U.EL _L is the uniform elongation in the L direction,
U.EL _C is uniform elongation in the C direction, U.EL _D is uniform elongation in the D direction.   In addition to the above composition, Cr: 0.05% to 1.00%, Mo: 0.05% to 0.50%, Ni: 0.05% to 1.00%, Cu: 0.05% to 1.00% in mass%. The high-strength hot-dip galvanized steel sheet according to claim 1, comprising one or more kinds.   3. In addition to the composition, the composition further contains one or two selected from V: 0.005% to 0.100% and Nb: 0.005% to 0.100% by mass%. The high-strength hot-dip galvanized steel sheet described in 1.   In addition to the above-mentioned composition, Ca: 0.0003% or more and 0.0050% or less, Mg: 0.0003% or more and 0.0050% or less, and REM: 0.0003% or more and 0.0050% or less in mass%, The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3.   The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 4, wherein the hot-dip galvanized layer is an alloyed galvanized layer. % By mass
C: 0.05% to 0.20%, Si: 0.8% to 2.0%,
Mn: 2.0% to 3.0%, P: 0.001% to 0.050%,
S: 0.0001% to 0.0100%, Al: 0.001% to 0.300%,
N: 0.0005% to 0.0100%, Ti: 0.005% to 0.050%,
B: A steel slab containing 0.0003% or more and 0.0050% or less, with the balance being composed of Fe and inevitable impurities, is heated to 1050 ° C or higher, and the finish rolling finish temperature is 800 ° C or higher and 950 ° C or lower. After the hot rolling is completed, cooling is started within 2 s, cooled to 600 ° C. or less at an average cooling rate of 50 ° C./s or more, and wound in a temperature range of 400 ° C. to 550 ° C. A hot-rolled sheet, the hot-rolled sheet, after pickling, cold-rolled at a rolling reduction of 30% or more to form a cold-rolled sheet, and the cold-rolled sheet at an average temperature increase rate of 5 ° C / s or more Primary temperature increase to a temperature range of 580 ° C to 720 ° C, and secondary temperature increase to a temperature range of 750 ° C to 900 ° C at an average temperature increase rate of 2 ° C / s or less following the primary temperature increase After maintaining the temperature in the temperature range of 750 ° C to 900 ° C for 15s to 600s, the average cooling rate in the temperature range of 750 ° C to 550 ° C is set to 5 ° C / s to 450 ° C to 550 ° C Cooled to the temperature range, by applying a galvanizing treatment, an area ratio of 30% or more of ferrite, which has an area ratio of 20% or more of martensite, the sum of the area ratio of the ferrite and the martensite is 98 The ferrite has an average crystal grain size of 10 μm or less, has a structure satisfying the following formula (1), further satisfies the following formulas (2) and (3), and has a TS of 780 MPa or more. A method for producing a high-strength hot-dip galvanized steel sheet having excellent fatigue properties and ductility, and having a small in-plane anisotropy of ductility, characterized by obtaining a hot-dip galvanized steel sheet.
Record
| (AR _L + AR _C −2 × AR _D ) / 2 | <0.2 (1)
| (EL _L + EL _C −2 × EL _D ) / 2 | <1.0 (2)
| (U.EL _L + U.EL _C −2 × U.EL _D ) / 2 | <0.6 (3)
However, AR _L ferrite crystal grains of the aspect ratio of the L cross section,
AR _C ferrite grains having an aspect ratio of C cross-section,
AR _D ferrite grains having an aspect ratio of D cross-section,
EL _L is the total elongation in the L direction, EL _C is the total elongation in the C direction,
EL _D is the total elongation in the D direction, U.EL _L is the uniform elongation in the L direction,
U.EL _C is uniform elongation in the C direction, U.EL _D is uniform elongation in the D direction.   In addition to the above composition, Cr: 0.05% to 1.00%, Mo: 0.05% to 0.50%, Ni: 0.05% to 1.00%, Cu: 0.05% to 1.00% in mass%. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 6, comprising one or more kinds.   8. In addition to the above composition, the composition further comprises one or two selected from V: 0.005% to 0.100% and Nb: 0.005% to 0.100% by mass%. A method for producing a high-strength hot-dip galvanized steel sheet as described in 1.   In addition to the above-mentioned composition, Ca: 0.0003% or more and 0.0050% or less, Mg: 0.0003% or more and 0.0050% or less, and REM: 0.0003% or more and 0.0050% or less in mass%, The method for producing a high-strength hot-dip galvanized steel sheet according to any one of claims 6 to 8.   The high-strength hot-dip galvanizing according to any one of claims 6 to 9, wherein after the hot-dip galvanizing treatment, an alloying treatment of hot-dip galvanizing is performed in a temperature range of 470 ° C to 600 ° C. A method of manufacturing a steel sheet.