JP4325233B2 - Composite structure type high-tensile cold-rolled steel sheet and hot-dip galvanized steel sheet excellent in deep drawability and strain age hardenability, and methods for producing them - Google Patents

Description

【００９９】
【表２】

【０１００】
表２に示す結果から、本発明例は、いずれも、引張り強さ(TS)が440MPa以上を満足するとともに、低い降伏比（YR）、高い伸び（El）、高いｒ値および高い歪時効硬化性（ＢＨＴ）を示して、深絞り成形性と歪時効硬化性に優れている。これに対し、本発明の範囲を外れる比較例では、降伏比（YR）が高いか、伸び（El）が低いか、ｒ値が低いか、あるいは歪時効硬化性が低くなっている。
【０１０１】
（試験例２）
表３に示す組成の溶鋼を転炉で溶製し、試験例1と同じ条件によって製造した冷延鋼帯（冷延板）に、連続溶融亜鉛めっきラインで、表４に示す焼鈍温度で再結晶焼鈍および溶融亜鉛めっきを施した。ここで、溶融亜鉛めっき条件は、めっき浴のAl濃度を0.13質量％とし、めっき浴温を460〜480℃とした。
【０１０２】
また、ここで、鋼板No.33および34に関しては、冷間圧延後に連続焼鈍ラインにて850℃で焼鈍を行った後、連続溶融亜鉛めっきライン内にて酸洗した後、焼鈍および溶融亜鉛めっきを施した。得られた鋼帯（溶融亜鉛めっき鋼板）に、さらに伸び率：0.8％の調質圧延を施した。
【０１０３】
さらに、鋼板No.ＯＯに関しては、溶融亜鉛めっき後、500℃に再加熱して合金化処理を行い、その後冷却した。合金化処理後の冷却に際して、300℃までの冷却速度は20℃／ｓであった。これらの結果を表４に示す。
【０１０４】
【表３】

【０１０５】
【表４】

【０１０６】
表４に示す結果から、本発明例は、いずれも、引張り強さ(TS)が440MPa以上を満足するとともに、低い降伏比（YR）、高い伸び（El）、高いｒ値および高い歪時効硬化性（ＢＨＴ）を示して、深絞り成形性と歪時効硬化性に優れている。これに対し、本発明の範囲を外れる比較例では、降伏比（YR）が高いか、伸び（El）が低いか、ｒ値が低いか、あるいは歪時効硬化性が低くなっている。
【０１０７】
【発明の効果】
本発明によれば、優れた深絞り成形性と歪時効硬化性を有する冷延鋼板および溶融亜鉛めっき鋼板を、安定して製造することができるという産業上格段の効果を奏する。本発明の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】 r値およびＢＨＴと（Ｖ/51）／（Ｃ/12＋Ｎ／14）との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a composite structure type high-tensile cold-rolled steel sheet and a composite structure type high-tensile hot-dip galvanized steel sheet having a tensile strength of 440 MPa or more and excellent in deep drawability and strain age hardening, which are useful for use in automobile steel sheets, The present invention relates to a manufacturing method thereof. In the present invention, “excellent in strain age hardening” specifically refers to the strain aging when pre-deformed with a tensile strain of 10% and subjected to aging treatment at a temperature of 170 ° C. for 20 minutes. It means that the amount of increase in tensile strength (BHT) before and after treatment, that is, the value obtained by subtracting the tensile strength before pre-deformation from the tensile strength after aging treatment is 40 MPa or more.
[0002]
[Prior art]
In recent years, in connection with exhaust gas regulations due to global environmental conservation issues, the reduction of vehicle body weight has become a very important issue. Recently, in order to reduce the weight of the vehicle body, it has been studied to increase the strength of a steel plate for automobiles and reduce the thickness of the steel plate.
[0003]
Since many automotive body parts made of steel sheets are formed by press working, the cold-rolled steel sheets used are required to have excellent press formability. In order to improve the press formability, as the mechanical properties of the steel sheet, high Rankford value (r value), high ductility (El), and low yield stress (YS) are required.
[0004]
However, generally, when the strength of a steel sheet is increased, the r value and ductility are lowered, the press formability is deteriorated, the yield stress is increased, and the shape freezing property tends to be deteriorated.
[0005]
Recently, in order to protect an occupant in the event of a collision, importance is placed on the safety of the automobile body. For this reason, an improvement in impact resistance characteristics that is a measure of safety in the event of a collision is required. The higher the strength of the finished vehicle, the more advantageous for improving the impact resistance. Therefore, when forming automotive parts, cold-rolled steel sheets that have low strength, high ductility, excellent press formability, and high strength and excellent impact resistance properties are most strongly desired when finished products are produced. It was.
[0006]
On the other hand, automobile parts are also required to have high corrosion resistance depending on the application site. Conventionally, various surface-treated steel sheets having excellent corrosion resistance have been used as steel sheets for automobiles. Among them, hot-dip galvanized steel sheets manufactured in continuous hot-dip galvanizing equipment that performs recrystallization annealing and plating on the same line can be manufactured with excellent corrosion resistance and low cost, and are also alloyed by heat treatment of hot-dip zinc. Hot-dip galvanized steel sheets can also be manufactured, and are often used because of their excellent weldability and press formability in addition to corrosion resistance.
[0007]
Therefore, in order to further promote weight reduction and strengthening of automobile bodies, high-tensile hot-dip galvanized steel sheets with excellent corrosion resistance and excellent press formability have become indispensable materials for continuous hot-dip galvanizing lines. .
[0008]
In response to such a demand, for example, as disclosed in Patent Document 1, a steel sheet that has both press formability and high strength has been developed. This steel sheet is a paint bake hardening type steel sheet whose yield stress increases when a coating baking process including holding at a high temperature of 100 to 200 ° C. is performed after press working. In this steel sheet, the amount of C (solid C amount) that is finally present in a solid solution state is controlled within an appropriate range, and it is soft during press forming, ensures shape freezing and ductility, and is baked after press forming. At the time of processing, the remaining solid solution C adheres to the dislocations introduced during press forming, thereby preventing the dislocations from moving and increasing the yield stress.
[0009]
[Patent Document 1]
JP 57-76131 A
[0010]
However, in the paint bake-hardening type automotive steel sheet, although the yield stress can be increased, the tensile strength cannot increase it.
[0011]
Patent Document 2 contains C: 0.08 to 0.20 mass%, Mn: 1.5 to 3.5 mass%, has a component composition consisting of the balance Fe and inevitable impurities, and has a uniform structure with a ferrite content of 5% or less. A bake hardenable high-tensile cold-rolled steel sheet composed of bainite or bainite partially containing martensite is disclosed. The cold-rolled steel sheet described in Patent Document 2 rapidly cools a temperature range of 400 to 200 ° C. in the cooling process after continuous annealing, and then gradually cools the structure to change the structure from a conventional ferrite-based structure to a bainite-based structure. As a structure of the above, an attempt is made to obtain a high bake hardening amount which has not been conventionally obtained.
[0012]
[Patent Document 2]
Japanese Patent Publication No. 5-24979
[0013]
However, in the steel sheet described in Patent Document 2, the yield stress is increased after baking, and a high bake hardening amount that has not been obtained in the past can be obtained, but it cannot be increased by the tensile strength yet, and the impact resistance characteristics are reduced. There was a problem that improvement could not be expected.
[0014]
On the other hand, various high-strength steel sheets made of a composite structure of ferrite and martensite have been developed as high-tensile steel sheets with good press formability. Of these, the composite steel sheet produced by gas jet cooling after continuous annealing has particularly low yield stress and high ductility and excellent bake hardenability, but has a low r value and deep drawability. However, there was a drawback that it was inferior.
[0015]
For this reason, attempts have been made to improve the deep drawability by increasing the r value of the composite structure steel plate. For example, in Patent Document 3, after cold rolling, the recrystallization temperature to 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 is high and a low yield ratio cannot be obtained. This high yield stress steel sheet is not suitable for press forming and has the disadvantages that the shape freezing property of the pressed part is poor. As described above, a steel plate having a high yield stress is not suitable for press forming and has a drawback that the shape freezing property of the pressed part is poor.
[0016]
[Patent Document 3]
Japanese Patent Publication No.55-10650
[0017]
A method for improving the high yield stress is disclosed in Patent Document 4. In this method, box annealing is first performed in order to obtain a high r value. The temperature during box annealing is set to a two-phase region of ferrite (α) -austenite (γ), and Mn is changed from α phase to γ phase during soaking. Thicken. This Mn-concentrated phase preferentially becomes a γ phase during continuous annealing, and a mixed structure can be obtained even at a cooling rate comparable to that of a gas jet, and the yield stress is also low. However, this method requires a relatively high temperature and long-time box annealing in the α-γ two-phase region 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 r value and a low yield stress.
[0018]
[Patent Document 4]
JP-A-55-100934
[0019]
On the other hand, in Patent Document 5, 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 is a two-phase region of α-γ after cold rolling. After cooling to 870 ° C and cooling at an average cooling rate of 100 ° C / s, a composite-structure cold-rolled steel sheet with extremely high r-value and low yield stress of r = 1.61, YS = 224MPa, TS = 482MPa A technology that enables manufacture of the above is disclosed. However, since a high cooling rate of 100 ° C./s cannot be achieved by ordinary gas jet cooling, a water quenching facility is required. Furthermore, since the water-quenched cold-rolled steel sheet also reveals a problem of surface treatment, problems on the manufacturing equipment and the material remain.
[0020]
[Patent Document 5]
Japanese Patent Publication No. 1-35900
[0021]
As techniques for solving the above problems, Patent Documents 6 and 7 disclose techniques for producing a composite structure type high-tensile cold-rolled steel sheet and hot-dip galvanized steel sheet that are excellent in deep drawability.
[0022]
However, although excellent deep drawability at the time of press working is ensured, there remains a problem in that an increase in strength in a finished product after press molding cannot be obtained.
[0023]
[Patent Document 6]
Japanese Patent Laid-Open No. 2002-226941
[0024]
[Patent Document 7]
Japanese Patent Laid-Open No. 2002-226942
[0025]
[Problems to be solved by the invention]
The present invention advantageously solves the above problems, and by regulating the steel composition and production conditions, it has a low yield ratio and a high r value at the time of molding, and a strain age hardening ability that can ensure high strength after molding. An object of the present invention is to propose a composite structure type high-tensile cold-rolled steel sheet and hot-dip galvanized steel sheet having excellent deep drawability and strain age hardenability, and a method for stably producing these steel sheets.
[0026]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors have conducted intensive studies on the influence of alloy elements on the microstructure and recrystallization texture of cold-rolled steel sheets. As a result, by setting the C content to a low carbon range and containing appropriate amounts of N and V, the solid solution (C, N) is reduced as much as possible before the recrystallization annealing, and the {111} recrystallization texture The high r value can be obtained by developing the carbon, and subsequently the α-γ two-phase region is heated to dissolve the V-based carbonitride, thereby concentrating C in the austenite, and the subsequent cooling process By producing martensite and securing a large amount of solute N, it is possible to produce a high-strength cold-rolled steel sheet having a high strain age hardening ability and a high r-value and high deep drawability with excellent r-value. I found it.
[0027]
Here, the “composite-structure cold-rolled steel sheet” of the present invention has a main phase of a ferrite phase, and a main phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure. The term “hot-dip galvanized steel sheet” means a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the cold-rolled steel sheet.
[0028]
First, basic experimental results performed by the present inventors will be described.
The basic composition is C: 0.02%, Si: 0.02%, Mn: 2.0%, P: 0.03%, S: 0.005%, Al: 0.01%, N: 0.015%, and V is 0.03-0.38. Various sheet bars having different V contents added in a mass% range were heated to 1250 ° C and soaked, and then subjected to three-pass rolling so that the finish rolling finish temperature was 900 ° C. did. In addition, after finishing rolling, the heat | fever equivalent process of 650 degreeC x 1h was performed as coil winding-up 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 subjected to recrystallization annealing at 830 ° C. for 60 s and then cooled at a cooling rate of 30 ° C./s.
[0029]
About the obtained cold-rolled steel plate, 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 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}. The strain aging characteristics were 10% tensile strain, subjected to heat treatment at 170 ° C. for 20 minutes, and then subjected to a normal tensile test. The difference between the tensile strength of the specimens collected from these cold-rolled sheets and the tensile strength of the specimens subjected to heat treatment at 170 ° C. for 20 minutes after applying 10% tensile strain, BHT).
[0030]
FIG. 1 shows the relationship between the r value and BHT and (V / 51) / (C / 12 + N / 14). By making the V content an atomic ratio with C and N in the range of 0.5 to 2.0, a high BHT is obtained together with a high r value, and a composite structure type cold rolling having a high r value and excellent in strain age hardening. It became clear that steel sheets could be manufactured.
[0031]
In the cold-rolled steel sheet of the present invention, C and N are precipitated and fixed as V-based carbon / nitride before recrystallization annealing, so the amount of dissolved C and N is small, and the {111} recrystallization texture is strong. Since it develops, a high r value is obtained. On the other hand, annealing in the α-γ two-phase region dissolves the V-based carbon and nitride, and solid solution C concentrates abundantly in the austenite phase, so that austenite is martensite in the subsequent cooling process. It has been clarified that a complex structure of ferrite and martensite can be obtained easily by transformation, and that a large amount of solute N can be obtained, so that high strain age hardening ability can also be obtained.
[0032]
Here, conventionally, Ti and Nb have been mainly used as carbon / nitride-forming elements. However, the present inventors have obtained carbon / nitride in order to effectively obtain solute C by annealing in a high temperature range. We focused on V, which has higher solubility than Ti and Nb. That is, V-based charcoal / nitride dissolves more easily during high-temperature annealing than Ti charcoal / nitride and Nb charcoal / nitride. As a result, annealing in α-γ two-phase region causes austenite to martensite. It has been discovered that a sufficient amount of solute C can be obtained for transformation. In addition, it has also been clarified that this phenomenon can be obtained by adding Nb in a complex manner, although the V component occurs most remarkably.
[0033]
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.
[0034]
(1) By mass%: C: 0.01 to 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% V: 0.01 to 0.5%, and V, C and N are
0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14)
Satisfy the relationship and the rest Is Having a composition consisting of Fe and inevitable impurities, 80-99% of the area ratio for the whole tissue, A composite excellent in deep drawability and strain age hardenability, characterized by having a structure consisting of a ferrite phase as a main phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure. Structure type high-tensile cold-rolled steel sheet.
[0035]
(2) By mass%: C: 0.01 to 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% V: 0.01 to 0.5%, Nb: 0.001 to 0.3%, and V and Nb, C and N
0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N / 14)
Satisfy the relationship and the rest Is Having a composition consisting of Fe and inevitable impurities, 80-99% of the area ratio for the whole tissue, A composite excellent in deep drawability and strain age hardenability, characterized by having a structure consisting of a ferrite phase as a main phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure. Structure type high-tensile cold-rolled steel sheet.
[0036]
(3) The depth described in (1) or (2) above, wherein, in addition to the above composition, the composition further contains, in mass%, one or two of the following groups A and B: Composite structure type high-tensile cold-rolled steel sheet with excellent drawability and strain age hardening.
Record
Group A: 2.0% by mass or less of one or two of Cr and Mo in total
Group B: One or two of Cu and Ni in total 2.0% by mass or less
[0037]
(4) A composite structure excellent in deep drawability and strain age hardenability, comprising a hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to (1), (2) or (3) Type high tensile hot dip galvanized steel sheet.
[0038]
(5) In mass%, C: 0.01 to 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 to 0.5%, and V, C and N are
0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14)
Satisfy the relationship and the rest Is A steel slab having a composition composed of Fe and unavoidable impurities is hot-rolled, subsequently pickled, and then cold-rolled. _c1 Transformation point ~ (A _c3 A method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability and strain age hardenability, characterized by continuous annealing at an annealing temperature in the temperature range of the transformation point + 50 ° C.
[0039]
(6) By mass%: C: 0.01 to 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% V: 0.01 to 0.5%, Nb: 0.001 to 0.3%, and V and Nb, C and N
0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N / 14)
Satisfy the relationship and the rest Is A steel slab having a composition composed of Fe and unavoidable impurities is hot-rolled, subsequently pickled, and then cold-rolled. _c1 Transformation point ~ (A _c3 A method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability and strain age hardenability, characterized by continuous annealing at an annealing temperature in the temperature range of the transformation point + 50 ° C.
[0040]
(7) The deep drawing according to (5) or (6) above, wherein the steel slab further contains one group or two groups out of group A and group B shown below by mass%. For producing a high-strength cold-rolled steel sheet with excellent structure and strain age hardening.
Record
Group A: 2.0% by mass or less of one or two of Cr and Mo in total
Group B: One or two of Cu and Ni in total 2.0% by mass or less
[0041]
(8) By mass%: C: 0.01 to 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% V: 0.01 to 0.5%, and V, C and N are
0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14)
Satisfy the relationship and the rest Is A steel slab having a composition composed of Fe and inevitable impurities is hot-rolled, subsequently pickled, cold-rolled, and then subjected to a continuous hot-dip galvanizing step. _c1 Transformation point ~ (A _c3 Production of high-strength hot-dip galvanized steel sheets with excellent deep drawability and strain age hardenability, characterized by performing hot-dip galvanization after continuous annealing at an annealing temperature in the temperature range of the transformation point + 50 ° C) Method.
[0042]
(9) In mass%, C: 0.01 to 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 to 0.5%, Nb: 0.001 to 0.3%, and V and Nb, C and N
0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N / 14)
Satisfy the relationship and the rest Is A steel slab having a composition composed of Fe and inevitable impurities is hot-rolled, subsequently pickled, cold-rolled, and then subjected to a continuous hot-dip galvanizing step. _c1 Transformation point ~ (A _c3 Production of high-strength hot-dip galvanized steel sheets with excellent deep drawability and strain age hardenability, characterized by performing hot-dip galvanization after continuous annealing at an annealing temperature in the temperature range of the transformation point + 50 ° C) Method.
[0043]
(10) The deep drawing according to (8) or (9) above, wherein the steel slab further contains one group or two groups out of group A and group B shown below by mass%. For producing a high-strength hot-dip galvanized steel sheet with excellent structure and strain age hardening.
Record
Group A: 2.0% by mass or less of one or two of Cr and Mo in total
Group B: One or two of Cu and Ni in total 2.0% by mass or less
[0044]
(11) In the production method described in (8), (9) or (10) above, after cold rolling, prior to continuous annealing in the continuous hot dip galvanizing step, A _c1 Transformation point ~ (A _c3 Deep drawability and strain age hardenability, characterized by continuous annealing at an annealing temperature in the temperature range of the transformation point + 50 ° C, followed by pickling to remove the concentrated layer of steel components formed on the steel sheet surface. For producing a high-strength hot-dip galvanized steel sheet with excellent structure.
[0045]
DETAILED DESCRIPTION OF THE INVENTION
The steel sheet of the present invention is a composite structure type high-tensile cold-rolled steel sheet and hot-dip galvanized steel sheet that are excellent in deep drawability and strain age hardening.
[0046]
First, the structure of the steel sheet of the present invention will be described.
The cold-rolled steel sheet and the hot-dip galvanized steel sheet of the present invention have a composite structure of a ferrite phase that is a main phase and a second phase that includes a martensite phase of 1% or more in terms of the area ratio relative to the entire structure.
[0047]
In order to obtain a cold-rolled steel sheet and a hot-dip galvanized steel sheet having low yield stress (YS) and high ductility (El) and excellent deep drawability, in the present invention, the structure of the steel sheet is a ferrite phase which is a main phase. And a composite structure of the second phase including the martensite phase. The ferrite phase as the main phase should be 80% or more in area ratio . This is because if the ferrite phase is less than 80% in area ratio, it is difficult to ensure high ductility and the press formability tends to decrease. Further, when better ductility is required, the ferrite phase is preferably 85% or more by area ratio. In order to take advantage of the composite structure, the ferrite phase needs to be 99% or less.
[0048]
Moreover, as a 2nd phase, in this invention, it is necessary to contain a martensite phase 1% or more by the area ratio with respect to the whole structure | tissue. If the martensite phase is less than 1% in terms of area ratio, low yield stress (YS) and high ductility (El) cannot be satisfied at the same time. More preferably, the martensite phase is 3% or more by area ratio. 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 may be mixed with either, and is not particularly limited. However, these pearlite phase, bainite phase, and residual austenite phase make the martensite phase more effective, so the total of these phases is 50% or less in terms of area ratio with respect to the structure of the second phase. Is preferred.
[0049]
The cold-rolled steel sheet and the hot-dip galvanized steel sheet having the above-described structure are steel sheets excellent in deep drawability having a low yield stress and high ductility.
[0050]
Next, the reason for limiting the composition of the cold-rolled steel sheet and hot-dip galvanized steel sheet of the present invention will be described. In the following description, mass% is simply expressed as%.
[0051]
C: 0.01-0.05%
C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of ferrite and martensite. In the present invention, it is necessary to contain 0.01% or more from the viewpoint of forming a composite structure. On the other hand, a content exceeding 0.05% inhibits the development of {111} recrystallized texture and reduces deep drawability. For this reason, in this invention, C content was limited to 0.01 to 0.05%. From the viewpoint of deep drawability, it is preferably 0.03% or less.
[0052]
Si: 1.0% or less
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel sheet. If its content exceeds 1.0%, (Si, Mn) -based nitrides Is formed, and the strain age-hardening property is lowered by decreasing the solid solution N. For this reason, Si content was limited to 1.0% or less. In addition, in order to acquire the said effect, it is preferable that Si content shall be 0.02% or more.
[0053]
Mn: 3.0% or less
Mn has the effect of strengthening steel, and further has the effect of reducing the critical cooling rate at which a composite structure of ferrite and martensite is obtained, and promoting the formation of a composite structure of ferrite and martensite. It is preferable to contain the quantity according to the cooling rate after crystal annealing. Further, since Mn is also an effective element for preventing aging cracks due to S, it is preferable to contain an appropriate amount according to the amount of S contained. However, if the Mn content exceeds 3.0%, the deep drawability and weldability deteriorate. For this reason, in the present invention, the Mn content is limited to 3.0% or less. The Mn content is preferably 0.5% or more in order to significantly exhibit the above effects, and more preferably 1.5% or more.
[0054]
P: 0.10% or less
P has an action of strengthening steel and can be contained in a necessary amount according to a desired strength. However, when the P content exceeds 0.10%, press formability deteriorates. For this reason, P content was limited to 0.10% or less. In addition, when more excellent press formability is required, the P content is preferably 0.08% or less. Moreover, in order to ensure intensity | strength, it is preferable that the content shall be 0.01% or more.
[0055]
S: 0.02% or less
S is present as an inclusion in the steel sheet, 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. Accordingly, in the present invention, the upper limit of the S content is 0.02%. When more excellent stretch flange formability is required, the S content is preferably 0.01% or less, more preferably 0.005% or less.
[0056]
Al: 0.03% or less
Al is added as a deoxidizing element for steel and is an element useful for improving the cleanliness of steel. However, if it is contained in an amount exceeding 0.03%, it forms AlN and is effective for strain age hardening. Molten N decreases. Therefore, the Al content is limited to 0.03% or less. In addition, it is preferable to contain 0.005% or more of Al from the viewpoint of exerting the deoxidizing action of steel and improving the cleanliness of the steel. Further, in the present invention, it does not exclude a melting method by a deoxidation method other than Al deoxidation, for example, Ti deoxidation or Si deoxidation may be performed, and a steel plate by these deoxidation methods is also of the present invention. Included in the range. At that time, even if Ca, REM, etc. are 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.
[0057]
N: 0.005-0.03%
N is one of the extremely important elements in the present invention, and is necessary for ensuring high strain age hardening. If the N content is less than 0.005%, high strain age hardenability cannot be obtained. On the other hand, if the N content exceeds 0.03%, the effect is saturated and nitride increases in the steel sheet, thereby Deep drawability is significantly degraded. For this reason, N content was limited to 0.005-0.03%.
[0058]
V: 0.01 to 0.5% and 0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14)
V is one of the most important elements in the present invention. Before recrystallization, solute C and N are precipitated and fixed as V-based carbon / nitride to develop a {111} recrystallization texture. In addition, when the α-γ two-phase annealing is performed, the V-based coal / nitride is dissolved to concentrate a large amount of the solid solution C into the austenite phase, and it is easy in the subsequent cooling process. When martensitic transformation is performed, a steel sheet having a composite structure of ferrite and martensite is obtained, and a large amount of solute N is secured to obtain high strain age hardening. Such an effect is effective when the V content is 0.01% or more and the C and N contents are 0.5 × (C / 12 + N / 14) ≦ V / 51. On the other hand, when V / 51> 2 × (C / 12 + N / 14) with respect to the V content exceeding 0.5% or the C and N contents, the V-based carbon / nitride in the α-γ two-phase region Since dissolution is difficult to occur, a composite structure of ferrite and martensite cannot be obtained, and solid solution N cannot be secured. Therefore, the V content was 0.01 to 0.5% and was limited to 0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14).
[0059]
Further, in the present invention, in addition to the above composition, Nb: 0.001 to 0.3% is contained in mass%, and the V content is 0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × ( Instead of satisfying the relationship C / 12 + N / 14), the V and Nb content and the C and N content are 0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × ( It is preferable to satisfy the relationship C / 12 + N / 14).
[0060]
Nb: 0.001 to 0.3% and V, Nb, C, and N satisfy the relationship of 0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N / 14) thing
Nb is a carbide forming element. By adding it in combination with V, before recrystallization, solute C and N are precipitated and fixed as a composite carbon / nitride of V and Nb, so that {111} recrystallization texture To develop a high r-value, and further, when annealing in the α-γ two-phase region, V and Nb composite charcoal / nitrides are dissolved to concentrate a large amount of solid solution C into the austenite phase. Then, by performing martensite transformation in the subsequent cooling process, a ferrite and martensite composite steel sheet can be obtained, and a large amount of solid solution N can be secured to obtain high strain age hardenability. Such an effect is effective when the Nb content is 0.001% or more and the C, N, and V contents are 0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93). On the other hand, in the case of (V / 51 + Nb / 93)> 2 × (C / 12 + N / 14) with respect to the Nb content exceeding 0.3% or the C, N and V contents, in the α-γ two-phase region Since the dissolution of carbon / nitride is difficult to occur, a composite structure of ferrite and martensite cannot be obtained, and a sufficient amount of solid solution N cannot be obtained. Therefore, the Nb content is in the range of 0.001 to 0.3%, and 0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N with respect to the C, N and V content. / 14).
[0061]
In the present invention, in addition to the above-described composition, the following groups A and B, namely,
Group A: 2.0% or less of one or two of Cr and Mo in total
Group B: 2.0% or less of one or two of Cu and Ni in total
It is preferable to contain 1 group or 2 groups of them.
[0062]
Group A: A total of 2.0% or less of one or two of Cr and Mo
Group A: Cr and Mo, like Mn, have the effect of reducing the critical cooling rate at which a composite structure of ferrite and martensite can be obtained and promoting the formation of a composite structure of ferrite and martensite. , If necessary. When one or two of Cr and Mo are contained in total exceeding 2.0%, deep drawability is lowered. For this reason, when it contains 1 type or 2 types of Group A: Cr and Mo, it is preferable to limit those content to 2.0% or less in total. In addition, in order to acquire the said effect, it is preferable to contain 0.05% or more of 1 type or 2 types in total in Cr and Mo.
[0063]
Group B: 2.0% or less of one or two of Cu and Ni in total
Group B: Cu, Ni has an action of strengthening steel, and can be contained in a necessary amount depending on the desired strength. However, when Cu and Ni are added alone or in total of more than 2.0%, Deep drawability tends to deteriorate. For this reason, Cu or Ni is preferably 2.0% or less in total of one or two of them. In addition, in order to acquire the said effect, it is preferable to contain 0.05% or more of 1 type or 2 types in total in Cu and Ni.
[0064]
In the present invention, the components other than those described above are not particularly limited, but there is no problem even if B, Ca, Zr, REM, etc. are contained within the range of the normal steel composition.
[0065]
The balance other than the above components is Fe and inevitable impurities. 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.
[0066]
Next, the manufacturing method of the cold-rolled steel plate and hot-dip galvanized steel plate 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 cold-rolled steel sheet, description of the reason for limiting the steel slab is omitted.
The cold-rolled steel sheet of the present invention comprises a steel slab having a composition within the above range as a raw material, hot-rolling the hot-rolled sheet by subjecting the raw material to hot rolling, and pickling the hot-rolled sheet. By sequentially performing a process, a cold rolling process in which the hot-rolled sheet is cold-rolled to form a cold-rolled sheet, and a recrystallization annealing process in which the cold-rolled sheet is subjected to recrystallization annealing to form a cold-rolled annealed sheet Manufactured.
[0067]
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.
[0068]
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.
[0069]
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 a decrease in yield 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.
[0070]
Finishing rolling finish temperature: 700 ℃ or more
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. In other words, if 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.
[0071]
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 scale loss. When the coiling temperature is less than 200 ° C., the shape of the steel sheet is significantly disturbed and the risk of causing problems in actual use increases. Therefore, the lower limit of the coiling temperature is more preferably 200 ° C.
[0072]
Thus, in the hot rolling process of the present invention, after the steel slab is heated to 900 ° C. or higher, the finish rolling finish temperature is hot rolled to 700 ° C. or higher, and the winding is performed at 800 ° C. or lower, preferably 200 ° C. or higher. It is preferable to take up at a taking temperature and to make a hot rolled sheet.
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.
[0073]
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.
[0074]
Next, the hot-rolled sheet is pickled to remove scale. The pickling step may be performed in accordance with a conventional method, and it is preferable to use, for example, hydrochloric acid or sulfuric acid-based processing solution as the pickling solution.
[0075]
Further, the hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet. 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} recrystallized texture hardly develops and it becomes difficult to obtain excellent deep drawability.
[0076]
Thereafter, in the recrystallization annealing step, the cold rolled sheet is subjected to recrystallization annealing to obtain a cold rolled annealed sheet. The recrystallization annealing is continuous annealing performed in a continuous annealing line or the like. The annealing temperature for recrystallization annealing is A _c1 Transformation point ~ (A _c3 It is necessary to carry out in the range from the α-γ two-phase region to the γ single-phase region in the temperature range of the transformation point + 50 ° C. Annealing temperature is A _c1 If it is lower than the transformation point, it becomes a ferrite single phase structure, and it becomes impossible to generate martensite. _c3 If the temperature is higher than the transformation point + 50 ° C., the crystal grains become coarse, and the {111} recrystallization texture does not develop and the deep drawability deteriorates significantly.
[0077]
A _c1 Transformation point and A _c3 The transformation point was determined by observation of an optical microscope structure of a sample obtained by subjecting a cold-rolled sheet to recrystallization annealing at a predetermined temperature for 60 seconds and then cooling at a cooling rate of 30 ° C./s. That is, the lowest temperature at which the steel sheet structure becomes a composite structure of ferrite phase and martensite phase is defined as A. _c1 A transformation point, highest temperature A _c3 It was defined as the transformation point.
[0078]
The cooling during recrystallization annealing is preferably performed at an average cooling rate from the annealing temperature to 300 ° C. of 5 ° C./s or more from the viewpoint of martensite formation.
[0079]
In addition, after the recrystallization annealing step, temper rolling with an elongation of 10% or less may be added for adjustment of shape correction, surface roughness, and the like.
[0080]
The cold-rolled steel sheet of the present invention can be applied not only as a cold-rolled steel sheet for processing but also as an original sheet of a surface-treated steel sheet for processing. Examples of the surface-treated steel sheet for processing include galvanized steel sheets (including alloy systems), tin-plated steel sheets, enamels and the like. Further, the cold-rolled steel sheet of the present invention may be subjected to a special treatment for improving chemical conversion treatment properties, weldability, press formability, corrosion resistance, and the like after galvanization.
[0081]
The hot-dip galvanized steel sheet of the present invention is a hot-rolling step using a steel slab having a composition within the above-mentioned range as a raw material, hot-rolling the raw material, and pickling the hot-rolled plate. A washing step, a cold rolling step in which the hot-rolled sheet is cold-rolled to form a cold-rolled plate, and a continuous hot-dip galvanizing step in which the cold-rolled plate is subjected to recrystallization annealing and hot-dip galvanizing to form a hot-dip galvanized steel sheet Are sequentially manufactured. In addition, a hot-rolling process, a pickling process, and a cold-rolling process are the same as the manufacturing method of said cold-rolled steel plate.
[0082]
In the continuous hot dip galvanizing process, recrystallization annealing and hot dip galvanizing are performed in a continuous hot dip galvanizing line to obtain a hot dip galvanized steel sheet. The annealing temperature for recrystallization annealing is A _c1 Transformation point ~ (A _c3 It is necessary to carry out in the range from the α-γ two-phase region to the γ single-phase region in the temperature range of the transformation point + 50 ° C. Annealing temperature is A _c1 Below the transformation point, a ferrite single-phase structure is formed, while (A _c3 At a high temperature exceeding the transformation point + 50 ° C., the crystal grains become coarse, and the {111} recrystallization texture does not develop and the deep drawability deteriorates significantly.
[0083]
After annealing, it is preferable to rapidly cool to a temperature range of 380 to 530 ° C. If the quenching stop temperature is less than 380 ° C., non-plating occurs, whereas if it exceeds 530 ° C., unevenness occurs on the plating surface, which is not preferable.
[0084]
The cooling rate is preferably 5 ° C./s or more in order to obtain a composite structure of a ferrite phase and a martensite phase.
[0085]
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 0.12 to 0.145% by mass. If the Al content in the plating bath is less than 0.12% by mass, alloying proceeds too much and the plating adhesion (powdering resistance) deteriorates. On the other hand, if it exceeds 0.145% by mass, non-plating occurs.
[0086]
Moreover, you may perform the alloying process of a plating layer after the hot dip galvanization process. At this time, it is preferable to perform alloying by heating subsequent to plating so that the Fe content in the plating layer is 9 to 12% by mass.
[0087]
In the alloying treatment, after the hot dip galvanizing treatment, the hot dip galvanized layer is alloyed by reheating to a temperature range of 450 to 550 ° C. Alloying at a high temperature exceeding 550 ° C makes it difficult to form a martensite phase and tends to lower the ductility of the steel sheet. On the other hand, when the alloying temperature is less than 450 ° C., the alloying progresses slowly and the productivity decreases. For this reason, the alloying temperature is preferably 450 to 550 ° C.
[0088]
After the alloying treatment, it is preferable to cool to 300 ° C. at a cooling rate of 5 ° C./s or more. When it is less than 5 ° C./s, it becomes difficult to form a martensite phase. For this reason, it is preferable to make the cooling rate in the temperature range from 300 degreeC after an alloying process into 5 degrees C / s or more.
[0089]
For the purpose of improving plating properties, after cold rolling, prior to continuous annealing in the continuous hot dip galvanizing process, A _c1 Transformation point ~ (A _c3 It is preferable to perform continuous annealing at an annealing temperature in the temperature range of (transformation point + 50 ° C.), and subsequently perform pickling to remove the concentrated layer of the steel components generated on the steel sheet surface. This is because the annealing promotes the concentration of the alloy elements on the steel sheet surface, and once the composite structure is formed in the continuous annealing line, the alloy elements are concentrated in the second phase.
[0090]
The annealing temperature in the continuous annealing line is _c1 It is preferable to carry out above the transformation point. However, the annealing temperature is (A _c3 If the temperature exceeds the transformation point + 50 ° C), the crystal grains become coarse. _c1 Transformation point ~ (A _c3 Even if annealing is performed in the temperature range of the transformation point + 50 ° C., deterioration of the material is inevitable, so the annealing temperature in the continuous annealing line is similar to the annealing temperature in the hot dip galvanizing process. _c1 Transformation point ~ (A _c3 The temperature range is preferably the transformation point + 50 ° C.
[0091]
The atmosphere of the continuous annealing line preferably has reducibility with respect to the steel sheet in order to prevent a large amount of scale from being generated, and generally H of about 1 to 10%. ₂ Nitrogen gas containing may be used.
[0092]
Steel sheets annealed in a continuous annealing line have P deposited on the steel sheet surface due to steel components, and Si, Mn, Cr, etc. are concentrated on the surface as oxides. It is preferable to remove by. In addition, you may perform the pickling which removes a surface concentrated layer in a continuous annealing line exit side, a continuous hot-dip galvanization line entrance side, or a normal pickling line.
[0093]
In addition, you may add the temper rolling for adjustment of shape correction, surface roughness, etc. to the steel plate after a plating process or an alloying process. Moreover, there is no inconvenience even if treatments such as resin or oil coating, various paintings or electroplating are performed.
[0094]
【Example】
(Test Example 1)
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. Next, after these steel slabs are heated to 1150 ° C, a hot-rolled steel strip with a thickness of 4.0 mm (hot) is applied by hot rolling at a finish rolling finish temperature of 900 ° C and a winding temperature of 650 ° C. 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.
[0095]
A _c1 Transformation point and A _c3 The transformation point is set at an annealing temperature of 650 to 900 ° C. at intervals of 5 ° C., and the cold-rolled sheet is subjected to recrystallization annealing at a predetermined temperature (annealing temperature) for 60 seconds and then cooled at 30 ° C./s. It was determined by observation of the optical microscope structure of the sample cooled at a speed. That is, the lowest temperature at which the steel sheet structure becomes a composite structure of ferrite phase and martensite phase is defined as A. _c1 A transformation point, highest temperature A _c3 It was defined as the transformation point.
[0096]
Subsequently, these cold-rolled steel strips (cold-rolled sheets) were subjected to recrystallization annealing at the annealing temperatures shown in Table 2 using a continuous annealing line. In the continuous annealing line, the average cooling rate from the annealing temperature to 300 ° C. was 10 to 20 ° C./s. The obtained cold-rolled steel strip (cold-rolled sheet) was further subjected to temper rolling with an elongation of 0.8%.
[0097]
A specimen is collected from the obtained steel strip, and the cross section (C cross section) orthogonal to the rolling direction is used to image the microstructure using an optical microscope or a scanning electron microscope, and in the main phase using an image analyzer. The structure fraction of a certain ferrite and the type and structure fraction of the second phase were determined. In addition, JIS No. 5 tensile test specimens were taken from the obtained steel strip with the direction perpendicular to the rolling direction as the longitudinal direction, and subjected to a tensile test in accordance with the provisions of JIS Z 2241, yield stress (YS), tensile strength. The thickness (TS), elongation (El), yield ratio (YR), and rankford value (r value) were determined. The strain aging characteristics were 10% tensile strain, subjected to heat treatment at 170 ° C. for 20 minutes, and then subjected to a normal tensile test. The difference between the tensile strength of the test piece taken from the cold-rolled sheet and the tensile strength of the test piece subjected to heat treatment at 170 ° C. for 20 minutes after imparting a tensile strain of 10% is the amount of increase in tensile strength after molding. (BHT). These results are shown in Table 2.
[0098]
[Table 1]

[0099]
[Table 2]

[0100]
From the results shown in Table 2, all of the inventive examples satisfy a tensile strength (TS) of 440 MPa or more, a low yield ratio (YR), a high elongation (El), a high r value, and a high strain age hardening. Property (BHT), and is excellent in deep drawability and strain age hardening. On the other hand, in the comparative example outside the scope of the present invention, the yield ratio (YR) is high, the elongation (El) is low, the r value is low, or the strain age hardening is low.
[0101]
(Test Example 2)
Molten steel having the composition shown in Table 3 was melted in a converter and re-applied to the cold-rolled steel strip (cold-rolled sheet) produced under the same conditions as in Test Example 1 at the annealing temperature shown in Table 4 on a continuous hot-dip galvanizing line. Crystal annealing and hot dip galvanizing were performed. Here, the hot dip galvanizing conditions were such that the Al concentration of the plating bath was 0.13 mass% and the plating bath temperature was 460 to 480 ° C.
[0102]
In addition, regarding steel plates No. 33 and 34, after annealing at 850 ° C. in a continuous annealing line after cold rolling, pickling in a continuous hot dip galvanizing line, and then annealing and hot dip galvanizing Was given. The obtained steel strip (hot dip galvanized steel sheet) was further subjected to temper rolling with an elongation of 0.8%.
[0103]
Furthermore, regarding the steel plate No. OO, after hot dip galvanization, it was reheated to 500 ° C. to perform alloying treatment, and then cooled. Upon cooling after the alloying treatment, the cooling rate to 300 ° C. was 20 ° C./s. These results are shown in Table 4.
[0104]
[Table 3]

[0105]
[Table 4]

[0106]
From the results shown in Table 4, all of the inventive examples satisfy a tensile strength (TS) of 440 MPa or more, a low yield ratio (YR), a high elongation (El), a high r value, and a high strain age hardening. Property (BHT), and is excellent in deep drawability and strain age hardening. On the other hand, in the comparative example outside the scope of the present invention, the yield ratio (YR) is high, the elongation (El) is low, the r value is low, or the strain age hardening is low.
[0107]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, there exists an industrial remarkable effect that the cold-rolled steel plate and hot-dip galvanized steel plate which have the outstanding deep drawing formability and strain age hardening property can be manufactured stably. When the 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 is a diagram showing a relationship between an r value and BHT and (V / 51) / (C / 12 + N / 14).

Claims (11)

C: 0.01 to 0.05% by mass%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 -0.5% and V, C and N are
0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14)
And the balance has a composition consisting of Fe and inevitable impurities,
80 to 99% in area ratio with respect to the entire structure, and having a structure consisting of a ferrite phase as a main phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure, Composite structure type high-tensile cold-rolled steel sheet with excellent deep drawability and strain age hardening. C: 0.01 to 0.05% by mass%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 -0.5% and Nb: 0.001-0.3%, and V and Nb and C and N
0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N / 14)
And the balance has a composition consisting of Fe and inevitable impurities,
80 to 99% in area ratio with respect to the entire structure, and having a structure consisting of a ferrite phase as a main phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure, Composite structure type high-tensile cold-rolled steel sheet with excellent deep drawability and strain age hardening. The deep drawability and strain age hardening according to claim 1 or 2, further comprising, in addition to the above composition, one or two of the following groups A and B: Composite structure high-tensile cold-rolled steel sheet with excellent properties.
Group A: One or two of Cr and Mo total 2.0% by mass or less Group B: One or two of Cu and Ni total 2.0% by mass or less   A composite structure type high-tensile hot-dip galvanized steel sheet excellent in deep drawability and strain age hardenability, comprising a hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to claim 1, 2 or 3. C: 0.01 to 0.05% by mass%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 -0.5% and V, C and N are
0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14)
A steel slab having a composition consisting of Fe and inevitable impurities is hot-rolled, and the steel slab is subsequently pickled and cold-rolled. Thereafter, the A _c1 transformation point to (A _c3 transformation point + 50 ° C. The method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability and strain age hardenability, characterized by continuous annealing at an annealing temperature in the temperature range. C: 0.01 to 0.05% by mass%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 -0.5% and Nb: 0.001-0.3%, and V and Nb and C and N
0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N / 14)
A steel slab having a composition consisting of Fe and inevitable impurities is hot-rolled, and the steel slab is subsequently pickled and cold-rolled. Thereafter, the A _c1 transformation point to (A _c3 transformation point + 50 ° C. The method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability and strain age hardenability, characterized by continuous annealing at an annealing temperature in the temperature range. The said steel slab is a mass%, and further contains 1 group or 2 groups of the following A group and B group, The deep drawability and strain age hardenability of Claim 5 or 6 characterized by the above-mentioned. For producing a high-strength cold-rolled steel sheet with excellent structure.
Group A: One or two of Cr and Mo total 2.0% by mass or less Group B: One or two of Cu and Ni total 2.0% by mass or less C: 0.01 to 0.05% by mass%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 -0.5% and V, C and N are
0.5 × (C / 12 + N / 14) ≦ V / 51 ≦ 2 × (C / 12 + N / 14)
The steel slab with the composition consisting of Fe and inevitable impurities is hot rolled, followed by pickling and cold rolling, and then the _Ac1 transformation point in the continuous hot dip galvanizing process. A composite structure type high-tensile molten zinc with excellent deep drawability and strain age-hardening properties, characterized by hot-dip galvanizing after continuous annealing at an annealing temperature in the temperature range of ~ ( _Ac3 transformation point + 50 ° C) Manufacturing method of plated steel sheet. C: 0.01 to 0.05% by mass%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.03% or less, N: 0.005 to 0.03% and V: 0.01 -0.5% and Nb: 0.001-0.3%, and V and Nb and C and N
0.5 × (C / 12 + N / 14) ≦ (V / 51 + Nb / 93) ≦ 2 × (C / 12 + N / 14)
The steel slab with the composition consisting of Fe and inevitable impurities is hot rolled, followed by pickling and cold rolling, and then the _Ac1 transformation point in the continuous hot dip galvanizing process. A composite structure type high-tensile molten zinc with excellent deep drawability and strain age-hardening properties, characterized by hot-dip galvanizing after continuous annealing at an annealing temperature in the temperature range of ~ ( _Ac3 transformation point + 50 ° C) Manufacturing method of plated steel sheet. 10. The deep drawability and strain age hardenability according to claim 8, wherein the steel slab further contains, in mass%, one or two of the following groups A and B: For producing a high-strength hot-dip galvanized steel sheet with excellent structure.
Group A: One or two of Cr and Mo total 2.0% by mass or less Group B: One or two of Cu and Ni total 2.0% by mass or less The manufacturing method according to claim 8, 9 or 10, wherein after cold rolling, prior to continuous annealing in the continuous hot dip galvanizing step, an annealing temperature in a temperature range from A _c1 transformation point to (A _c3 transformation point + 50 ° C). The composite structure type high-tensile hot dip galvanized steel with excellent deep drawability and strain age hardenability, which is characterized by performing continuous annealing at the top and subsequent pickling to remove the concentrated layer of steel components formed on the steel sheet surface. A method of manufacturing a steel sheet.

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