JP2004002909A - Complex metallographic structure type high tensile strength hot-dip galvanized cold rolled steel sheet with excellent deep drawability and stretch-flange formability, and manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a complex metallographic structure type high tensile strength hot-dip galvanized cold rolled steel sheet having excellent deep drawability and stretch-flange formability and to provide its manufacturing method. <P>SOLUTION: The complex metallographic structure type high tensile strength hot-dip galvanized cold rolled steel sheet has a composition which consists, by mass, of 0.01 to 0.05% C, 0.1 to 1.0% Si, 1.0 to 3.0% Mn, ≤0.10% P, ≤0.02% S, 0.005 to 0.1% Al, ≤0.02% N, 0.01 to 0.2% V, 0.001 to 0.2% Nb and the balance essentially Fe with inevitable impurities and in which the respective contents (mass%) of V, Nb and C satisfy a relation of 0.5×C/12≤(V/51+Nb/93)≤2×C/12. This steel sheet has a steel structure in which a primary phase is composed of a ferritic phase consisting of a polygonal ferrite phase and a bainitic ferrite phase and which has a secondary phase containing ≥1% by area ratio of a martensite phase. Further, this steel sheet has a hot-dip galvanized layer on the surface. <P>COPYRIGHT: (C)2004,JPO

Description

【００６９】
【表２】

【００７０】
表２に示す結果から、本発明例は、いずれも、目標とする、低い降伏比（ＹＲ≦７０％）、高い伸び（Ｅｌ≧２８％）、高いランクフォード値（ｒ≧１．３）および高い穴拡げ率（λ≧１００％）を有し、深絞り成形性に優れた鋼板となっている。特に本発明例では、２次連続焼鈍温度を、本発明の範囲である（Ａ_ｃ３変態点−５０℃）〜（Ａ_ｃ３変態点＋５０℃）の温度域とすることによって、穴拡げ率（λ）が飛躍的に上昇し、λ≧１００％以上を確保できる。これに対し、本発明の範囲を外れる条件で製造した比較例では、降伏比（ＹＲ）が高いか、伸び（Ｅｌ）、ランクフォード値（ｒ値）または穴拡げ率（λ）が低下した鋼板となっている。
【００７１】
【発明の効果】
本発明によれば、強度伸びバランスに優れるとともに、深絞り成形性および伸びフランジ性にも優れた溶融亜鉛めっき冷延鋼板を安定して製造することが可能となり、産業上格段の効果を奏する。本発明の溶融亜鉛めっき冷延鋼板を自動車部品に適用した場合、プレス成形が容易で、自動車車体の軽量化に十分に寄与できるという効果もある。
【図面の簡単な説明】
【図１】ＶとＮｂの含有量とＣとの関係を表す比（Ｖ／５１＋Ｎｂ／９３）／（Ｃ／１２）がランクフォード値（ｒ値）と強度伸びバランス（ＴＳ×Ｅｌ）に及ぼす影響を示した図である。
【図２】２次連続焼鈍温度が穴拡げ率（λ）に及ぼす影響を示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet having a tensile strength of 440 MPa or more, which is useful for using steel sheets for automobiles and the like, and a method for producing the same.
[0002]
[Prior art]
In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of conservation of the global environment. In addition, from the viewpoint of protecting occupants in the event of a vehicle collision, it is also required to improve the safety of the automobile body. For this reason, studies are being actively conducted to reduce the weight and strengthen the automobile body.
It is said that it is effective to increase the strength of component materials in order to satisfy the weight reduction and strengthening of the automobile body at the same time. Recently, high-tensile steel plates have been actively used for automobile parts.
[0003]
Since many automobile parts made of steel plates are formed by press working, the steel plates for automobiles are required to have excellent press formability. However, generally, when the strength of the steel sheet is increased, the ductility represented by the Rankford value (r value) and the elongation (El) is lowered, and the stretch flangeability is also lowered, so that the press formability is deteriorated. At the same time, the yield stress increases and the shape freezing property tends to deteriorate. In particular, the larger the so-called strength-elongation balance value expressed by the product TS × El of tensile strength (TS) and elongation (El), the more advantageous the press formability. Ductility has been attempted.
Steel sheets that have both high strength and high ductility include residual austenitic steel (residual γ steel) using strain-induced plasticity, dual-phase steel (DP steel) having two phases of polygonal ferrite and martensite. Development research on so-called composite structure steel is underway.
[0004]
On the other hand, since high corrosion resistance is also required for automobile parts depending on the application site, conventionally, various surface-treated steel sheets having excellent corrosion resistance have been used as steel sheets for automobile parts. Among such surface-treated steel sheets, hot-dip galvanized steel sheets manufactured in a continuous hot-dip galvanizing facility that performs recrystallization annealing and plating on the same line have excellent corrosion resistance and can be manufactured at low cost. An alloyed hot-dip galvanized steel sheet that is further heat-treated after galvanization can be produced, and is widely used because of its excellent weldability and press formability in addition to corrosion resistance.
[0005]
Therefore, in order to further promote weight reduction and strengthening of the automobile body, it is desired to develop a high-tensile hot-dip galvanized steel sheet having excellent corrosion resistance and press formability by a continuous alloying hot-dip galvanizing line.
[0006]
As a typical example of a high-tensile steel sheet having good press formability, a composite structure steel sheet composed of a composite structure of ferrite and martensite can be cited. Particularly, a composite structure steel sheet manufactured by gas jet cooling after continuous annealing has a yield stress ( YS) is low, and it has both high ductility and excellent bake hardenability. However, the continuous hot dip galvanizing line is generally installed with the annealing equipment and the plating equipment being made continuous. Due to the presence of this continuous plating process, cooling after annealing is interrupted at the plating temperature, and the average cooling rate throughout the process is necessarily reduced.
[0007]
Therefore, in a steel plate manufactured by a continuous hot dip galvanizing line, it is difficult to generate martensite generated under cooling conditions with a high cooling rate in the steel plate after hot dipping. For this reason, it is generally difficult to produce a high-tensile hot-dip galvanized steel sheet having a composite structure of ferrite and martensite in a continuous hot-dip galvanizing line.
Further, the above-mentioned composite structure steel plate has the disadvantages that the Rankford value (r value) is low and the deep drawability is inferior, the hole expansion ratio (λ) is low and the stretch flangeability is inferior.
[0008]
Under these unfavorable conditions, the production of high-strength hot-dip galvanized steel sheets with a high degree of hardenability, such as Cr and Mo, is used to produce a low-temperature transformation phase. A method for facilitating the process is common. However, there is a problem that adding a large amount of the above-described alloy element causes an increase in manufacturing cost.
[0009]
In addition, as disclosed in Japanese Patent Publication No. 62-40405, etc., by specifying the cooling rate after annealing in a continuous hot dip galvanizing line or cooling after plating, high strength of structure strengthened hot dip galvanizing A method of manufacturing a steel sheet has also been proposed. However, such a method may be difficult due to the restrictions on the equipment of the continuous hot dip galvanizing line, and further improvement has been desired for the ductility of the steel sheet obtained by this method.
[0010]
Furthermore, attempts have been made to improve the Rankford value (r value) of the composite structure steel plate. For example, in Japanese Patent Publication No. 55-10650, after cold rolling, a recrystallization temperature of ~ A _c3 A technique is disclosed in which box annealing is performed at the temperature of the transformation point, and then heating is performed at 700 to 800 ° C. to obtain a composite structure, followed by continuous annealing with quenching and tempering. However, in this method, since quenching and tempering is performed during continuous annealing, the yield stress YS is high and a low yield ratio YR cannot be obtained. Here, the yield ratio YR is the ratio of the yield stress YS to the tensile strength TS, and YR = YS / TS. This high yield stress steel sheet has the disadvantages that it is difficult to press form and the shape freezing property of the pressed parts is poor.
[0011]
A method for improving the high yield stress YS is disclosed in JP-A-55-100934. In this method, in order to obtain a high Rankford value (r value), box annealing is first performed. The temperature during box annealing is set to a two-phase region of ferrite (α) -austenite (γ), and from the α phase during soaking. Mn is concentrated in the γ phase. This Mn-concentrated phase preferentially becomes a γ phase during continuous annealing, a mixed structure can be obtained even at a cooling rate comparable to that of a gas jet, and the yield stress YS is also low. However, this method requires a relatively high temperature annealing in a relatively high temperature of two phases of α-γ for Mn concentration. Therefore, frequent adhesion between steel plates, generation of temper color, and furnace body inner cover There are a number of problems in the manufacturing process, such as a reduction in the service life. Conventionally, it has been difficult to industrially stably produce a high-tensile steel plate having such a high Rankford value (r value) and a low yield stress YS.
[0012]
In addition, in Japanese Examined Patent Publication No. 1-35900, a steel having a composition of 0.012 mass% C-0.32 mass% Si-0.53 mass% Mn-0.03 mass% P-0.051 mass% Ti. After cold rolling, heating to 870 ° C., which is a two-phase region of α-γ, and cooling at an average cooling rate of 100 ° C./s, r = 1.61, YS = 224 MPa, TS = 482 MPa A technique is disclosed that enables the production of a composite structure cold-rolled steel sheet having a very high Rankford value (r value) and low yield stress. However, since it is difficult to achieve a high cooling rate of 100 ° C./s with a normal continuous hot dip galvanizing line, water quenching equipment is required, and water quenching cold rolled steel sheet has a problem of surface treatment. However, there are problems in manufacturing equipment and materials.
[0013]
[Problems to be solved by the invention]
The present invention advantageously solves the above problems, and by regulating the steel composition, in particular, the contents of C, V, and Nb, and in particular, the annealing temperature as a production condition, it has an excellent strength-elongation balance and has a high rank. In addition to the Ford value, the objective is to propose a composite-structured high-tensile hot-dip galvanized cold-rolled steel sheet that is excellent in stretch flangeability and a technology that can be stably manufactured. The “hot-dip galvanized cold-rolled steel sheet” as used in the present invention is a so-called non-alloyed hot-dip galvanized cold-rolled steel sheet that is not subjected to heat alloying treatment after hot-dip galvanization and heat-alloying treatment after hot-dip galvanization. It means both so-called galvannealed steel sheets.
[0014]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors have determined the alloy elements that affect the microstructure and recrystallization texture of the hot-dip galvanized cold-rolled steel sheet having a hot-dip galvanized layer on the surface of the cold-rolled steel sheet, and the annealing temperature conditions. We have earnestly studied the influence. As a result, by setting the C content to 0.01 to 0.05% by mass and containing the appropriate amounts of V and Nb, before recrystallization annealing, the solid solution C is reduced as much as possible to {111} By developing the crystal texture, a high Rankford value (r value) can be obtained, and by heating in a temperature range of 750 to 950 ° C. by primary continuous annealing, V and Nb carbides are dissolved. Thus, a large amount of solid solution C can be generated, and the secondary annealing temperature can be subsequently set to (A _c3 Transformation point −50 ° C.) to (A _c3 By making the transformation point + 50 ° C), C is concentrated in the austenite and a martensite phase is generated in the subsequent cooling process, so that it has excellent ductility, high rankford value, elongation It was found that a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet with good flangeability can be produced.
[0015]
Here, the composite structure type hot-dip galvanized cold-rolled steel sheet, which is the steel of the present invention, has a main phase of ferrite phase, which is bainitic with high dislocation density generated by cooling process from polygonal ferrite and austenite region. It is a hot-dip galvanized cold-rolled steel sheet having a composite structure with a second phase containing a martensite phase with an area ratio of 1% or more, in which a ferrite phase is mixed.
[0016]
First, basic experimental results performed by the present inventors will be described.
By mass%, C: 0.02%, Si: 0.5%, Mn: 2.0%, P: 0.08%, S: 0.005%, Al: 0.03%, N: 0.00. By adding 002% as a basic composition and adding V in the range of 0.05 to 0.10% by mass and Nb in the range of 0.001 to 0.16% by mass, different V and Nb contents are obtained. The various steel materials were heated to 1250 ° C. and soaked, and then subjected to three-pass rolling so that the finish rolling finish temperature was 880 ° C. to a plate thickness of 4.0 mm. In addition, after finishing rolling, a heat retention equivalent process of 650 ° C. × 3 h was performed as a coil winding process. Subsequently, cold rolling with a rolling reduction of 70% was performed to a sheet thickness of 1.2 mm. Subsequently, these cold-rolled sheets were heated to 880 ° C. and then subjected to primary continuous annealing (recrystallization annealing) that was cooled to 400 ° C. or lower at an average cooling rate of 15 ° C./s. Then (A _c3 Transformation point −50 ° C.) to (A _c3 The secondary continuous annealing is performed by heating to 850 ° C. which is within the temperature range of the transformation point + 50 ° C. and then cooling to the temperature range of 450 to 500 ° C. at an average cooling rate of 15 ° C./s, and then 0.13 mass of Al. After immersing in a hot dip galvanizing bath containing 5%, alloying treatment in a temperature range of 450 to 550 ° C. (Fe content in the plating layer: about 10% by mass) is performed, and then 15 ° C./s. Cooled to room temperature at an average cooling rate.
[0017]
About the obtained hot-dip galvanized steel sheet, the tensile test was implemented and the tensile characteristic was investigated. The tensile test was performed using a JIS No. 5 tensile test piece. The tensile strength TS and ductility El are values when a tensile test is performed in a direction perpendicular to the rolling direction. The r value is the rolling direction (r _L ), 45 degree direction (r _D ) And the direction perpendicular to the rolling direction (90 degrees) (r _c ) Average r value {= (r _L + R _c + 2 × r _D ) / 4}.
[0018]
FIG. 1 is a graph showing the influence of the contents of V and Nb on the r value and the strength elongation balance (TS × El) in relation to C, and the horizontal axis represents the contents of V and Nb and the C content. The atomic ratio ((V / 51 + Nb / 93) / (C / 12)), and the vertical axis shows the r value and the strength elongation balance (TS × El) separately.
[0019]
From FIG. 1, by limiting the content of V and Nb in the steel to an atomic ratio with C within a range of 0.5 to 2.0, a high r value and a high strength elongation balance can be obtained. It became clear that a composite structure type hot-dip galvanized cold-rolled steel sheet having high value and high ductility El can be produced.
[0020]
Next, (V / 51 + Nb / 93) / (C / 12) = 1.1 steel material (A / A) among the hot-dip galvanized cold-rolled steel sheets used in FIG. _c3 Transformation point: 890 ° C.), followed by pickling, followed by cold rolling, followed by heating to 850 ° C. and then cooling to 400 ° C. at an average cooling rate of 15 ° C./s. (Recrystallization annealing), after heating to a temperature range of 760 to 960 ° C. and then cooling at an average cooling rate of 10 ° C./s to a temperature range of 450 to 500 ° C., Al is reduced to 0. After immersion and plating in a hot dip galvanizing bath containing 13% by mass, an alloying treatment in a temperature range of 450 to 550 ° C. (Fe content in the plating layer: about 10% by mass) is performed, and then average cooling is performed. By cooling to room temperature at a rate of 15 ° C./s, the obtained hot-dip galvanized steel sheet was subjected to a hole expansion test to obtain a hole expansion ratio (λ) and to evaluate stretch flangeability.
[0021]
In the hole expansion test, a punch hole was formed in the specimen by punching with a 10 mmφ punch in accordance with the provisions of JFST 1001, and then using a conical punch with an apex angle of 60 °, the flash was on the outside, Hole expansion was performed until a crack penetrating the plate thickness occurred, and the hole expansion ratio λ was obtained. The hole expansion ratio λ is λ (%) = {(d−d ₀ ) / D ₀ } It calculated | required in x100. D ₀ : Initial hole inner diameter, d: hole inner diameter when cracking occurs.
[0022]
FIG. 2 is a diagram showing the influence of the secondary continuous annealing temperature on the hole expansion rate (λ). Secondary annealing temperature (A _c3 Transformation point −50 ° C.) to (A _c3 It has been clarified that, by setting the transformation point to + 50 ° C., a high hole expansion ratio can be obtained, and a composite structure type hot-dip galvanized steel sheet excellent in stretch flangeability can be produced.
[0023]
In the hot-dip galvanized cold-rolled steel sheet of the present invention, the {111} recrystallized texture is strongly developed in the primary continuous annealing process, since there is little solid solution C and N before recrystallization annealing, and a high Rankford value. In addition, V and Nb carbides are dissolved after recrystallization, and solid solution C is concentrated in the austenite phase in a large amount. As a result, the austenite phase is transformed into a martensite phase in the subsequent cooling process. A composite structure of a ferrite phase and a martensite phase having a value can be obtained. This cold-rolled steel sheet is further subjected to (A _c3 Transformation point −50 ° C.) to (A _c3 (Transformation point + 50 ° C.) is transformed into an austenite phase in which a large amount of solid solution C is concentrated, an austenite phase and a ferrite phase in which a small amount of solid solution C is present, and a large amount of solid solution C is concentrated in the subsequent cooling process. The transformed austenite phase is transformed into a martensite phase, and the austenite phase with less solid solution C is transformed into a bainitic ferrite phase with a high dislocation density, so that the main phase is a polygonal ferrite and bainitic ferrite phase. A composite structure in which the two phases are martensite phases is obtained. The composite structure steel sheet having such a structure has a high hole expansion ratio because the difference in hardness between the polygonal and bainitic ferrite phases of the main phase and the martensite phase of the second phase is small. Though possible, details are not clear.
[0024]
The present invention has been completed by further study based on the above-described findings, and the gist of the present invention is as follows.
(1) By mass%
C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% or less, Al : 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2% and Nb: 0.001 to 0.2%, and V, Nb and C And the content (mass%)
0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12
The remainder has a component composition consisting essentially of Fe and inevitable impurities, the main phase is a ferrite phase consisting of a polygonal ferrite phase and a bainitic ferrite phase, and the area ratio is 1% or more Composite structure type high-tensile hot dip galvanizing with excellent deep drawability and stretch flangeability, characterized by having a steel structure having a second phase containing a martensite phase and having a hot dip galvanized layer on the surface Cold rolled steel sheet.
[0025]
(2) In mass%
C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% or less, Al : 0.005-0.1%, N: 0.02% or less, V: 0.01-0.2%, Nb: 0.001-0.2% and Ti: 0.001-0.3% And the content (% by mass) of V, Nb and Ti and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12
The remainder has a component composition consisting essentially of Fe and inevitable impurities, the main phase is a ferrite phase consisting of a polygonal ferrite phase and a bainitic ferrite phase, and the area ratio is 1% or more Composite structure type high-tensile hot dip galvanizing with excellent deep drawability and stretch flangeability, characterized by having a steel structure having a second phase containing a martensite phase and having a hot dip galvanized layer on the surface Cold rolled steel sheet.
[0026]
(3) In addition to the above composition, Mo: 0.01 to 0.5% by mass is further included, and is excellent in deep drawability and stretch flangeability as described in (1) or (2) above Composite type high-tensile hot-dip galvanized cold-rolled steel sheet.
[0027]
(4) In mass%
C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% or less, Al : 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2% and Nb: 0.001 to 0.2%, and V, Nb and C And the content (mass%)
0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, subsequently pickled, then cold-rolled, and then heated to 750 ° C. or more and 950 ° C. or less, and then at an average cooling rate of 5 ° C./s or more. Apply primary continuous annealing to cool to 400 ° C or lower, then (A _c3 Transformation point −50 ° C.) to (A _c3 A method for producing a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet excellent in deep drawability and stretch flangeability, characterized by performing hot-dip galvanization after secondary continuous annealing at a transformation point + 50 ° C.
[0028]
(5) By mass%
C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% or less, Al : 0.005-0.1%, N: 0.02% or less, V: 0.01-0.2%, Nb: 0.001-0.2% and Ti: 0.001-0.3% And the content (% by mass) of V, Nb and Ti and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, subsequently pickled, then cold-rolled, and then heated to 750 ° C. or more and 950 ° C. or less, and then at an average cooling rate of 5 ° C./s or more. Apply primary continuous annealing to cool to 400 ° C or lower, then (A _c3 Transformation point −50 ° C.) to (A _c3 A method for producing a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet excellent in deep drawability and stretch flangeability, characterized by performing hot-dip galvanization after secondary continuous annealing at a transformation point + 50 ° C.
[0029]
(6) The steel slab further contains Mo: 0.01 to 0.5% by mass in addition to the above composition, and the deep drawability and stretch flange according to the above (4) or (5) For producing a composite structure type high-tensile hot-dip galvanized steel sheet having excellent properties.
[0030]
(7) The above (4), (5), characterized in that a pickling treatment is performed between the primary continuous annealing and the secondary continuous annealing to remove a concentrated layer of steel components generated on the steel sheet surface. Or the manufacturing method of the composite structure type | mold high tension hot-dip galvanized cold-rolled steel plate excellent in the deep drawability and stretch flangeability as described in (6).
[0031]
DETAILED DESCRIPTION OF THE INVENTION
The hot-dip galvanized steel sheet of the present invention is a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet excellent in deep drawability and stretch flangeability with a tensile strength TS of 440 MPa or more.
[0032]
First, the structure of the steel sheet of the present invention will be described.
The structure of the hot-dip galvanized cold-rolled steel sheet of the present invention is a composite structure of a main phase in which polygonal ferrite having a low dislocation density and a bainitic ferrite phase having a high dislocation density are mixed, and a second phase containing a martensite phase. Have. In addition, the polygonal ferrite and bainitic ferrite phase, which are the main phases, have developed {111} textures and have high Rankford values.
[0033]
In order to obtain a hot-dip galvanized cold-rolled steel sheet having a low yield stress (YS) and a high strength-elongation balance (TS × El) and having excellent deep drawability and stretch flangeability, the structure of the cold-rolled steel sheet is used in the present invention. Needs to be a composite structure of a main phase which is a ferrite phase composed of a polygonal ferrite phase and a bainitic ferrite phase and a second phase including a martensite phase. Polygonal ferrite phase and bainitic ferrite phase that are main phases should be 80% or more in area ratio, and the bainitic ferrite phase in the main phase should be included in the area ratio of 5% or more with respect to the entire structure. preferable. In the present invention, the polygonal ferrite phase is contained in an area ratio of about 40% or more with respect to the entire structure. This is because if the polygonal ferrite phase and bainitic ferrite phase are less than 80% in terms of area ratio, it is difficult to ensure a high strength-elongation balance and press formability tends to decrease. Moreover, when better ductility and hole expansibility are required, the proportion of the bainitic ferrite phase in the main phase is preferably 10% or more in terms of area ratio. In order to utilize the advantages of the composite structure, the polygonal ferrite phase and bainitic ferrite phase which are the main phases are preferably 99% or less.
[0034]
Further, as the second phase, in the present invention, the martensite phase needs to be present, and the steel sheet of the present invention is a composite structure steel containing the martensite phase in an area ratio of 1% or more with respect to the entire structure. is there. If the martensite phase is less than 1% in terms of area ratio, it is difficult to simultaneously satisfy a low yield ratio (YR) and a high strength-elongation balance (TS × El). The second phase may be composed of a martensite phase with an area ratio of 1% or more alone, or a martensite phase with an area ratio of 1% or more and other pearlite, bainite, and retained austenite phases as subphases. It is good also as a mixture with either.
[0035]
Next, the reason for limiting the composition of the hot dip galvanized cold rolled steel sheet of the present invention will be described. The mass% is simply written as%.
C: 0.01 to 0.05%
C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of a polygonal ferrite phase as a main phase and a bainitic ferrite phase and a martensite phase as a second phase. It is necessary to contain 0.01% or more from the viewpoint of tissue formation. On the other hand, the content exceeding 0.05% inhibits the development of {111} recrystallized texture and lowers the deep drawability and hole expandability. For this reason, in this invention, C content was limited to 0.01 to 0.05%.
[0036]
Si: 0.1 to 1.0% or less
Si is a useful strengthening element that can increase the strength of the steel sheet, that is, improve the strength-elongation balance, without significantly reducing the ductility of the steel sheet. To obtain this effect, the Si content is 0. Must be 1% or more. However, if the Si content exceeds 1.0%, the surface properties, particularly the plating properties, deteriorate. For this reason, Si content was limited to 0.1 to 1.0%. From the viewpoint of plating properties, the Si content is more preferably less than 0.7%.
[0037]
Mn: 1.0-3.0%
Mn has an action of strengthening steel, and further, a critical cooling rate at which a composite structure of a ferrite phase composed of a polygonal ferrite phase and a bainitic ferrite phase as main phases and a martensite phase as a second phase is obtained. And has a function of promoting the formation of a composite structure of a ferrite phase composed of a polygonal ferrite phase and a bainitic ferrite phase, which are main phases, and a martensite phase, which is a second phase. It is preferably contained depending on the cooling rate. A martensite phase is not generated at a slow cooling rate below the critical cooling rate, and a bainite phase or a pearlite phase is generated instead. However, when no martensite is present in the second phase, the strength-elongation balance tends to decrease. It is in. Therefore, the addition of Mn is effective for facilitating the formation of the martensite phase, that is, for reducing the critical cooling rate. Mn is an effective element for preventing hot cracking due to S, and is preferably contained according to the amount of S contained. Such an effect becomes remarkable when Mn is contained by 1.0% or more. On the other hand, if the Mn content exceeds 3.0%, the deep drawability and weldability deteriorate. For this reason, in this invention, Mn content was limited to 1.0 to 3.0% of range.
[0038]
P: 0.10% or less
P has the effect | action which strengthens steel, and can be contained suitably according to desired intensity | strength, However, When P content exceeds 0.10%, intensity | strength elongation balance will fall and deep drawability will deteriorate. For this reason, the P content is limited to 0.10% or less. In addition, when more excellent press formability is required, the P content is preferably 0.08% or less. In addition, in order to acquire the said effect, it is preferable to contain P 0.005% or more.
[0039]
S: 0.02% or less
S is an element present in the steel sheet as an inclusion, and is an element that causes deterioration of the ductility and formability of the steel sheet, particularly stretch flangeability. Therefore, it is preferable to reduce it as much as possible. In the present invention, the upper limit of the S content is 0.02%. In addition, when the more excellent stretch flangeability is requested | required, it is preferable that S content shall be 0.01% or less, More preferably, it is 0.005% or less.
[0040]
Al: 0.005 to 0.1%
Al is added as a deoxidizing element for steel, and is an element useful for improving the cleanliness of steel. However, if it is less than 0.005%, there is no effect of addition, while it exceeds 0.1%. Even so, no further deoxidation effect can be obtained, and conversely the deep drawability deteriorates. For this reason, Al content was limited to 0.005-0.1%. In the present invention, a melting method by a deoxidation method other than Al deoxidation is not excluded. For example, Ti deoxidation or Si deoxidation may be performed. Included in the range. At that time, even if Ca, REM, or the like is added to the molten steel, the characteristics of the steel sheet of the present invention are not inhibited at all, and it is a matter of course that a steel sheet containing Ca, REM, etc. is also included in the scope of the present invention.
[0041]
N: 0.02% or less
N is an element that increases the strength of the steel sheet by solid solution strengthening or strain age hardening. However, when it exceeds 0.02%, nitride increases in the steel sheet, and thereby the deep drawability of the steel sheet is remarkable. It deteriorates to. For this reason, N was limited to 0.02% or less. In addition, when improvement of press formability is requested | required more, it is preferable to reduce N, and it is suitable to set it as 0.004% or less.
[0042]
V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and satisfy the relationship of 0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12.
V and Nb are the most important elements in the present invention, and the {111} recrystallized texture is developed by precipitating and fixing solute C as V and Nb-based carbides before recrystallization, thereby increasing the rank of Ford. A value can be obtained. Further, during annealing, V and Nb-based carbides are dissolved so that a large amount of solid solution C is concentrated in the austenite phase, and then the martensite transformation is performed in the subsequent cooling process. A steel sheet having a composite structure of a ferrite phase composed of a ferrite phase and a martensite phase as a second phase is obtained. In order to achieve such an effect, the contents of V and Nb are 0.01% or more and 0.001% or more, respectively, and the contents (mass%) of C, V and Nb are 0.5 × C / It is necessary to satisfy the relationship of 12 ≦ (V / 51 + Nb / 93). On the other hand, the content of at least one of V and Nb exceeds 0.2%, or the content (mass%) of C, V, and Nb is (V / 51 + Nb / 93)> 2 × C / 12 And the dissolution of V and Nb-based carbides during annealing hardly occur, so that the composite phase of the ferrite phase consisting of the polygonal ferrite phase and bainitic ferrite phase as the main phase and the martensite phase as the second phase Cannot be obtained. Therefore, in the present invention, V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and 0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12. It was limited to satisfying the relationship.
[0043]
Further, in the present invention, in addition to the above composition, it is preferable to contain Ti: 0.001 to 0.3% by mass%. In this case, the contents of C, V, and Nb (mass) %), Which is 0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12, the relational expression of the contents of C, V, Nb and Ti (% by mass). That is, it is necessary to satisfy the relational expression of 0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12.
Ti is a carbide forming element. Before recrystallization, solid solution C is precipitated and fixed as V, Nb and Ti carbides, thereby developing a {111} recrystallized texture and obtaining a high Rankford value. Furthermore, during annealing, V, Nb, and Ti-based carbides are dissolved to concentrate a large amount of solid solution C into the austenite phase, and then the martensitic transformation is performed in the subsequent cooling process, whereby the polygonal ferrite phase that is the main phase and A steel sheet having a composite structure of a ferrite phase composed of a bainitic ferrite phase and a martensite phase as a second phase is obtained. In order to achieve such an effect, it is necessary that the Ti content is 0.001% or more and the relationship of 0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) is satisfied. On the other hand, if the Ti content exceeds 0.3% or (V / 51 + Nb / 93 + Ti / 48)> 2 × C / 12, it is difficult for carbides to dissolve during annealing. A composite structure of a ferrite phase composed of a certain polygonal ferrite phase and bainitic ferrite phase and a martensite phase as the second phase cannot be obtained. Therefore, when Ti is contained, Ti is 0.001 to 0.3% and satisfies the relationship of 0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12. Limited to that.
[0044]
Moreover, in this invention, it is preferable to contain Mo: 0.01-0.5% further in addition to the above-mentioned composition.
Mo: 0.01 to 0.5%
Mo, like Mn, reduces the critical cooling rate at which a composite structure of a ferrite phase composed of a polygonal ferrite phase and a bainitic ferrite phase, which are main phases, and a martensite phase, which is a second phase, is obtained. It has an action of promoting the formation of a composite structure of a phase and a martensite phase, and can be contained as necessary. The effect is exhibited by the inclusion of 0.01% or more of Mo. However, when the Mo content exceeds 0.5%, the deep drawability deteriorates, so the Mo content is limited to 0.01 to 0.5%.
[0045]
In the present invention, the balance other than the above-described components is preferably substantially composed of Fe and inevitable impurities, but B, Ca, REM, etc. are included within the range of the normal steel composition. There is no problem.
[0046]
B is an element having an effect of improving the hardenability of steel and can be contained as necessary. However, if the B content exceeds 0.003%, the effect is saturated, so B is preferably 0.003% or less. A more desirable range is 0.0001 to 0.002%. Ca and REM have the effect | action which controls the form of a sulfide type inclusion, and it has the effect which improves the stretch flangeability of a steel plate by this. Such an effect is saturated when the content of one or two selected from Ca and REM exceeds 0.01% in total. For this reason, it is preferable that the content of one or two of Ca and REM is 0.01% or less in total. A more preferable range is 0.001 to 0.005%.
[0047]
Other inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn : 0.01% or less, Co: 0.1% or less.
[0048]
Next, the manufacturing method of the hot dip galvanized cold rolled steel sheet of this invention is demonstrated.
Since the composition of the steel slab used in the production method of the present invention is the same as that of the above-described hot-dip galvanized cold-rolled steel sheet, description of the reason for limiting the steel slab is omitted.
The hot-dip galvanized cold-rolled steel sheet of the present invention comprises a steel slab having a composition within the above-described range as a raw material, hot-rolling the raw material into a hot-rolled sheet, and pickling the hot-rolled sheet. Pickling process, cold rolling process to cold-roll the hot-rolled sheet to make a cold-rolled sheet, continuous annealing process to recrystallize the cold-rolled sheet, and hot-dip zinc by annealing and hot-dip galvanizing It is manufactured by sequentially performing a continuous hot dip galvanizing step to form a plated steel sheet. Moreover, the process of performing the pickling which removes the concentrated layer of the component in steel produced | generated on the steel plate surface is given between a continuous annealing process and a continuous hot-dip galvanization process as needed.
[0049]
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab casting method. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then heating it again, without cooling, the method of inserting it into a heating furnace as it is, or after performing a slight heat retention Energy-saving processes such as direct feed rolling and direct rolling, which are immediately rolled, can be applied without problems.
[0050]
The above-mentioned raw material (steel slab) is heated and subjected to a hot rolling step of hot rolling to obtain a hot rolled sheet. The hot rolling process only needs to be a condition that enables the production of a hot rolled sheet having a desired thickness, and there is no particular problem even if normal rolling conditions are used. For reference, suitable hot rolling conditions are shown below.
[0051]
Slab heating temperature: 900 ℃ or more
The slab heating temperature is preferably low because the precipitates are coarsened to develop a {111} recrystallized texture and improve deep drawability. However, if the heating temperature is less than 900 ° C., the rolling load increases and the risk of trouble occurring during hot rolling increases. For this reason, it is preferable that slab heating temperature shall be 900 degreeC or more. In addition, the upper limit of the slab heating temperature is more preferably 1300 ° C. due to an increase in scale loss accompanying an increase in oxidized weight. Needless to say, using a so-called sheet bar heater that heats the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling is of course effective.
[0052]
Finishing rolling finish temperature: 700 ° C or higher
The finish rolling finish temperature (FDT) is preferably 700 ° C. or higher in order to obtain a uniform hot-rolled base metal structure that provides excellent deep drawability after cold rolling and recrystallization annealing. That is, when the finish rolling finish temperature is less than 700 ° C., the hot rolled base metal structure becomes non-uniform, the rolling load during hot rolling increases, and the risk of trouble occurring during hot rolling increases. is there.
[0053]
Winding temperature: 800 ℃ or less
The winding temperature is preferably 800 ° C. or lower. That is, when the coiling temperature exceeds 800 ° C., the scale increases and the yield tends to decrease due to the scale loss. If the coiling temperature is less than 200 ° C., the shape of the steel sheet is remarkably disturbed and the risk of causing problems in actual use increases. Therefore, the lower limit of the coiling temperature is more preferably 200 ° C.
[0054]
As described above, in the hot rolling step of the present invention, after the steel slab is heated to 900 ° C. or higher, the hot rolling at a finish rolling finish temperature of 700 ° C. or higher is performed, and the winding at 800 ° C. or lower, preferably 200 ° C. or higher is performed. It is preferable to use a hot-rolled sheet wound at temperature.
In the hot rolling process of the present invention, in order to reduce the rolling load during hot rolling, lubrication rolling may be performed between some or all passes of finish rolling. In addition, performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.10-0.25.
[0055]
Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
[0056]
Next, the hot-rolled sheet is pickled and then cold-rolled to obtain a cold-rolled sheet. Pickling may be performed under normal conditions. The cold rolling condition is not particularly limited as long as it can be a cold-rolled sheet having a desired size and shape, but the rolling reduction during cold rolling is preferably 40% or more. This is because if the rolling reduction is less than 40%, the {111} recrystallization texture does not develop and it becomes difficult to obtain excellent deep drawability.
[0057]
Then, the primary continuous annealing process which recrystallizes and anneals the said cold-rolled steel plate to make a cold-rolled annealing plate is performed. Recrystallization annealing is performed in a continuous annealing line. The annealing temperature for recrystallization annealing needs to be performed in a temperature range of 750 ° C. or more and 950 ° C. or less. When the annealing temperature is less than 750 ° C., a ferrite single phase structure is formed, and V and Nb carbides are not sufficiently dissolved. Even after the subsequent hot dip galvanizing treatment, a composite structure of a ferrite phase composed of a polygonal ferrite phase and a bainitic ferrite phase as main phases and a martensite phase as a second phase cannot be obtained. On the other hand, at a high temperature exceeding 950 ° C., the stretch flange characteristics and the like deteriorate.
[0058]
In the recrystallization annealing, it is necessary to cool from the annealing temperature to 400 ° C. or less at an average cooling rate of 5 ° C./s or more from the viewpoint of martensite formation. If the average cooling rate is less than 5 ° C./s, the martensite phase is difficult to be formed, and a structure composed of a ferrite phase and a pearlite phase, or a structure composed of a ferrite phase and a bainite phase is formed, and the strength-elongation balance decreases. Therefore, in the present invention, since the presence of the second phase containing the martensite phase is essential, it is necessary to set the average cooling rate to 5 ° C./s or more which is equal to or higher than the critical cooling rate. .
[0059]
Next, the cold-rolled steel sheet is subjected to secondary continuous annealing in a continuous hot-dip galvanizing line, and then hot-dip galvanized to give a hot-dip galvanized cold-rolled steel sheet. The secondary continuous annealing temperature is (A _c3 Transformation point −50 ° C.) to (A _c3 It is necessary to carry out in the temperature range of transformation point + 50 ° C. Secondary continuous annealing temperature is (A _c3 If the transformation point is less than −50 ° C., bainitic ferrite is difficult to form, and therefore the bainitic ferrite structure necessary for improving hole expansibility cannot be obtained. On the other hand, (A _c3 This is because in the temperature range exceeding the transformation point + 50 ° C., the crystal grains become coarse and the {111} recrystallization texture does not develop and the deep drawability deteriorates remarkably.
[0060]
After the secondary continuous annealing, it is preferable to rapidly cool to a temperature range of 380 to 530 ° C., which is a hot dip galvanizing treatment temperature. If the quenching stop temperature is less than 380 ° C., non-plating is likely to occur, and if it exceeds 530 ° C., unevenness is likely to occur on the plating surface, which is not preferable. The cooling rate is from a secondary continuous annealing temperature to a hot dip galvanizing temperature in order to obtain a composite structure of the polygonal ferrite phase and bainitic ferrite phase as the main phase and the martensite phase as the second phase. It is preferable to rapidly cool at an average cooling rate of 5 ° C./s or more. After the rapid cooling, it is subsequently immersed in a hot dip galvanizing bath and hot dip galvanized. At this time, the Al concentration of the plating bath is preferably in the range of 0.12 to 0.145 mass%. This is because when the Al content in the plating bath is less than 0.12 mass%, alloying tends to proceed excessively and the plating adhesion (powdering resistance) tends to deteriorate, while on the other hand, when the Al content exceeds 0.145 mass%. This is because non-plating is likely to occur.
[0061]
Moreover, you may perform the alloying process of a plating layer after the hot dip galvanization process. In addition, when performing an alloying process, it is preferable to implement so that Fe content rate in a plating layer may be 9 to 12%.
[0062]
In the alloying treatment, it is preferable to reheat to a temperature range of 450 to 550 ° C. to alloy the hot dip galvanized layer after the hot dip galvanizing treatment. After the alloying treatment, it is preferable to cool to at least 300 ° C. at an average cooling rate of 5 ° C./s or more. Alloying at a high temperature exceeding 550 ° C makes it difficult to form a martensite phase and may reduce the ductility of the steel sheet. On the other hand, if the alloying temperature is lower than 450 ° C, the alloying progresses slowly and the productivity decreases. Because there is a tendency to.
[0063]
Further, when the cooling rate after the alloying treatment is extremely small, it becomes difficult to form a martensite phase. For this reason, it is preferable to make the average cooling rate in the temperature range from 300 degreeC after an alloying process into 5 degrees C / s or more.
[0064]
In addition, when it is necessary to further improve the plating property, a pickling treatment is performed between the primary continuous annealing and the secondary continuous annealing to remove the concentrated layer of steel components generated on the steel sheet surface. Is preferred. The steel sheet annealed in the primary continuous annealing line has a surface concentration because P, which is a component in the steel, precipitates on the surface of the steel sheet and Si, Mn, Cr, etc. tend to concentrate as oxides. This is because it is preferable to remove the layer by pickling in terms of improving the plating property. Then, secondary continuous annealing which is reduction annealing is performed in a continuous hot dip galvanizing line. In addition, you may perform pickling in the pickling tank installed in the continuous hot-dip galvanizing line.
[0065]
Further, the steel plate after the plating treatment or the alloying treatment may be subjected to temper rolling for adjusting the shape correction, surface roughness, and the like. Moreover, there is no inconvenience even if treatments such as resin or oil coating, various paintings or electroplating are performed.
[0066]
【Example】
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. Then, after heating these steel slabs to 1250 ° C., a hot rolling process of hot rolling at a finish rolling finish temperature of 880 ° C. and a winding temperature of 650 ° C. Hot rolled sheet). Subsequently, a cold-rolled steel strip (cold-rolled sheet) having a thickness of 1.2 mm was obtained by a cold-rolling process in which the hot-rolled steel strip (hot-rolled sheet) was pickled and cold-rolled. Subsequently, primary cold annealing was performed on these cold-rolled steel strips (cold-rolled sheets) under the conditions shown in Table 2 in a continuous annealing line. In addition, the cooling rate in the primary continuous annealing shown in Table 2 was an average cooling rate up to 300 ° C. Subsequently, secondary continuous annealing was performed on the continuous hot dip galvanizing line under the conditions shown in Table 2, followed by hot dip galvanizing and alloying treatment. Here, the hot dip galvanizing treatment is cooled to 380 to 530 ° C. which is the hot dip galvanizing treatment temperature, and then immersed and plated in a hot dip galvanizing bath containing 0.13% by mass of Al, followed by 450 to 550 ° C. Alloying treatment (Fe content in the plating layer: about 10% by mass) in the temperature range was applied, and then cooled to 300 ° C. or less at an average cooling rate up to 300 ° C. of 15 ° C./s. In addition, the cooling rate in the secondary continuous annealing shown in Table 2 was an average cooling rate up to the hot dip galvanizing treatment. In addition, some steel strips (steel plates No. 1 and 3 in Table 2) were subjected to pickling treatment in a continuous hot-dip galvanizing line after primary continuous annealing and then subjected to secondary continuous annealing. It was. The obtained steel strip (hot-dip galvanized cold-rolled steel sheet) was further subjected to temper rolling with an elongation of 0.5%.
[0067]
A specimen is taken from the obtained steel strip, and a microscopic structure is imaged at a magnification of about 400 to 1000 times using an optical microscope or a scanning electron microscope with respect to a cross section (L cross section) parallel to the rolling direction. Using an analyzer, the structure fraction of the main phase polygonal ferrite and bainitic ferrite and the type and structure fraction of the second phase were determined. In addition, from the obtained steel strip, a JIS No. 5 tensile test piece was sampled in the same manner as when the above-mentioned basic experimental results were obtained, and a tensile test was conducted in accordance with the provisions of JIS Z 2241 to obtain a yield stress (YS ), Tensile strength (TS), elongation (El), and yield ratio (YR). Moreover, the r value calculated | required the average r value (average plastic strain ratio) based on the prescription | regulation of JISZ2254 using the JIS5 tension test piece extract | collected from the obtained steel strip, and made this r value. Further, the hole expansion rate (λ) was also obtained. Here, the hole expansion rate (λ) was obtained by collecting a test piece from the obtained steel strip and performing a hole expansion test in accordance with the provisions of JFST 1001 as described above. These results are shown in Table 2.
[0068]
[Table 1]

[0069]
[Table 2]

[0070]
From the results shown in Table 2, all of the inventive examples are aimed at low yield ratio (YR ≦ 70%), high elongation (El ≧ 28%), high Rankford value (r ≧ 1.3) and The steel sheet has a high hole expansion ratio (λ ≧ 100%) and is excellent in deep drawability. Particularly in the present invention example, the secondary continuous annealing temperature is within the scope of the present invention (A _c3 Transformation point −50 ° C.) to (A _c3 By setting the temperature range to the transformation point + 50 ° C., the hole expansion rate (λ) increases dramatically, and λ ≧ 100% or more can be secured. On the other hand, in the comparative example manufactured under conditions outside the scope of the present invention, the steel sheet having a high yield ratio (YR) or a low elongation (El), rankford value (r value), or hole expansion ratio (λ). It has become.
[0071]
【The invention's effect】
According to the present invention, it is possible to stably produce a hot-dip galvanized cold-rolled steel sheet having excellent strength-elongation balance and excellent deep-drawing formability and stretch flangeability. When the hot-dip galvanized cold-rolled steel sheet of the present invention is applied to automobile parts, press forming is easy, and there is an effect that it can sufficiently contribute to weight reduction of an automobile body.
[Brief description of the drawings]
FIG. 1 shows the ratio (V / 51 + Nb / 93) / (C / 12) representing the relationship between the content of V and Nb and C, which influences the Rankford value (r value) and the strength-elongation balance (TS × El). It is the figure which showed the influence.
FIG. 2 is a graph showing the influence of secondary continuous annealing temperature on the hole expansion rate (λ).

Claims (7)

C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% in mass% Hereinafter, Al: 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2% and Nb: 0.001 to 0.2%, and V and Content (mass%) of Nb and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12
The remainder has a component composition consisting essentially of Fe and inevitable impurities, the main phase is a ferrite phase consisting of a polygonal ferrite phase and a bainitic ferrite phase, and the area ratio is 1% or more Composite structure type high-tensile hot dip galvanizing with excellent deep drawability and stretch flangeability, characterized by having a steel structure having a second phase containing a martensite phase and having a hot dip galvanized layer on the surface Cold rolled steel sheet. C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% in mass% Hereinafter, Al: 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and Ti: 0.001 to 0 .3%, and the content (mass%) of V, Nb and Ti and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12
The remainder has a component composition consisting essentially of Fe and inevitable impurities, the main phase is a ferrite phase consisting of a polygonal ferrite phase and a bainitic ferrite phase, and the area ratio is 1% or more Composite structure type high-tensile hot dip galvanizing with excellent deep drawability and stretch flangeability, characterized by having a steel structure having a second phase containing a martensite phase and having a hot dip galvanized layer on the surface Cold rolled steel sheet. In addition to the above composition, Mo: 0.01 to 0.5 mass% is further contained, and the composite structure type high-tensile fusion excellent in deep drawability and stretch flangeability according to claim 1 or 2 Galvanized cold rolled steel sheet. C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% in mass% Hereinafter, Al: 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2% and Nb: 0.001 to 0.2%, and V and Content (mass%) of Nb and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, subsequently pickled, then cold-rolled, and then heated to 750 ° C. or more and 950 ° C. or less, and then at an average cooling rate of 5 ° C./s or more. It is characterized in that it is subjected to primary continuous annealing to cool to 400 ° C. or lower, and then subjected to secondary continuous annealing at ( _Ac3 transformation point−50 ° C.) to ( _Ac3 transformation point + 50 ° C.) and then hot dip galvanizing. A method for producing a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet excellent in deep drawability and stretch flangeability. C: 0.01 to 0.05%, Si: 0.1 to 1.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% in mass% Hereinafter, Al: 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and Ti: 0.001 to 0 .3%, and the content (mass%) of V, Nb and Ti and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, subsequently pickled, then cold-rolled, and then heated to 750 ° C. or more and 950 ° C. or less, and then at an average cooling rate of 5 ° C./s or more. It is characterized in that it is subjected to primary continuous annealing to cool to 400 ° C. or lower, and then subjected to secondary continuous annealing at ( _Ac3 transformation point−50 ° C.) to ( _Ac3 transformation point + 50 ° C.) and then hot dip galvanizing. A method for producing a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet excellent in deep drawability and stretch flangeability. Steel slab contains Mo: 0.01-0.5 mass% further in addition to the said composition, The composite structure excellent in the deep drawability and stretch flangeability of Claim 4 or 5 characterized by the above-mentioned. Type high-tensile hot-dip galvanized steel sheet. 7. The depth according to claim 4, 5 or 6, wherein a pickling treatment is performed between the primary continuous annealing and the secondary continuous annealing to remove the concentrated layer of the steel components generated on the steel sheet surface. A method for producing a composite structure type high-tensile hot-dip galvanized cold-rolled steel sheet excellent in drawability and stretch flangeability.

Images