JP2004052071A - High tensile strength cold rolled steel sheet with composite structure having excellent stretch flanging property, strength-ductility balance and strain age hardenability, and method of producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet with a composite structure which has TS (Tensile Strength) of ≥440 MPa, and has excellent a strength-ductility balance and strain age hardenability, and to provide a method of producing the same. <P>SOLUTION: A slab comprising 0.01 to 0.10% C and 0.0050 to 0.0250% N, also satisfying ≥0.3 N/Al, and furthercomprising C, N, Mn, and Si so that 12(C+N)+Mn-Si satisfies 0.1 to 1.5 is produced. Next, the slab is subjected to a hot rolling stage wherein it is heated to ≥1,000°C, is thereafter subjected to rough rolling and finish rolling at an outlet side temperature of ≥800°C and is coiled at ≤750°C; a cold rolling stage; and an annealing stage wherein it is subjected to annealing of heating to an annealing temperature of (Ac<SB>1</SB>+20°C) to (Ac<SB>3</SB>+50°C), is subjected to first cooling wherein CR (Cooling Rate) from the annealing temperature to a prescribed temperature (cooling finishing temperature) in the range of (Ac<SB>1</SB>+20°C) to (Ac<SB>3</SB>-100°C) is 5 to 50°C/s, is subjected to secondary cooling wherein CR from the cooling finishing temperature to 100°C is ≥300°C/s, and is tempered at 150 to 450°C. These stages are successively performed. <P>COPYRIGHT: (C)2004,JPO

Description

【００６３】
【表２】

【００６４】
【表３】

【００６５】
本発明例は、いずれも、４４０ＭＰａ以上の引張強さＴＳを有し、高い穴拡げ率λ、高い強度−延性バランスＴＳ×Ｅｌ、高いＢＨ量、および高いΔＴＳを示し、伸びフランジ性、強度−延性バランス、歪時効硬化特性に優れた高張力冷延鋼板となっている。これに対し、本発明範囲を外れる比較例では、伸びフランジ性、強度−延性バランス、歪時効硬化特性のいずれかが低い値となっている。
【００６６】
【発明の効果】
以上のように、本発明によれば、４４０ＭＰａ以上の引張強さを有し、伸びフランジ性、強度−延性バランス、歪時効硬化特性が同時に優れ、自動車用部品として好適な高張力冷延鋼板を、安価にしかも安定して製造でき、産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】強度−延性バランスＴＳ×Ｅｌと｛１２（Ｃ＋Ｎ）＋Ｍｎ−Ｓｉ　｝との関係を示すグラフである。
【図２】穴拡げ率λと｛１２（Ｃ＋Ｎ）＋Ｍｎ−Ｓｉ　｝との関係を示すグラフである。
【図３】ＢＨ量、ΔＴＳと｛１２（Ｃ＋Ｎ）＋Ｍｎ−Ｓｉ　｝との関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-tensile cold-rolled steel sheet having a tensile strength of 440 MPa or more, which is suitable mainly for use in automobile body parts and the like, and particularly relates to a high-strength steel excellent in stretch flangeability, strength-ductility balance and strain age hardening properties. The present invention relates to a cold-rolled steel sheet and a method for producing the same.
In the present invention, “excellent in stretch flangeability” means that the hole expansion ratio λ is 100% or more, and “excellent in strength-ductility balance” means that tensile strength TS and elongation are excellent. The product of El and the strength-ductility balance TS × El are 17000 MPa% or more, and “excellent in strain aging hardening properties” means that after pre-deformation with a tensile strain of 5%, it is heated at a temperature of 170 ° C. for 20 minutes. When the aging treatment is performed under the conditions for holding, the amount of deformation stress increase before and after this aging treatment (hereinafter referred to as BH amount: BH amount = (yield stress after aging treatment) − (pre-deformation stress before aging treatment)) is 100 MPa or more. And the increase in tensile strength before and after the strain aging treatment (pre-deformation + aging treatment) (denoted by ΔTS: ΔTS = (tensile strength after aging treatment)-(tensile strength before pre-deformation) ) Is 60 MPa or more. .
[0002]
[Prior art]
In recent years, there has been a demand for improving fuel efficiency of automobiles from the viewpoint of preserving the global environment and improving the safety of automobile bodies from the viewpoint of protecting occupants in the event of a vehicle collision. In order to respond to such demands, studies for both reducing the weight and strengthening the vehicle body are being actively pursued. According to these studies, it is said that it is effective to increase the strength of component materials in order to simultaneously satisfy the demands for weight reduction and strengthening of automobile bodies. Is used regularly.
[0003]
However, since many automotive body parts made of steel sheets are formed by press working, high-strength steel sheets used for body parts are required to have excellent press formability. Therefore, as the mechanical properties of the steel sheet, it is required to have high stretch flangeability (hole expansion ratio λ), high strength-ductility balance (TS × El), and high strain aging characteristics (high BH amount, high ΔTS). Has been.
[0004]
As a typical example of a high-strength steel sheet having good press-formability, a composite-structure high-strength steel sheet having a structure in which soft ferrite and hard martensite are combined is exemplified. In particular, a composite-structure high-strength steel sheet manufactured by subjecting a cold-rolled steel sheet to continuous annealing and then performing gas jet cooling is a steel sheet having low yield stress, high ductility, and also having bake hardenability. However, this type of composite structure type steel sheet is generally good in formability under normal conditions, but has a problem in forming under severe conditions due to poor stretch flange formability. There is also a problem that bake hardenability is not so high.
[0005]
In recent years, as a steel sheet that can simultaneously satisfy good press formability and high strength after forming, it is soft and easy to press form before press forming, and after press forming, it can be hardened by paint baking treatment to increase component strength. And BH steel sheets have been developed.
As an example of such a BH steel sheet, for example, Japanese Unexamined Patent Publication No. 55-141526 discloses that at least Nb is added in accordance with the content of C, N, and Al in steel, and Nb / (solid solution C + A method for manufacturing a cold-rolled steel sheet in which the solid solution C and the solid solution N in the steel sheet are adjusted by adjusting the solution N) within a specific range and further controlling the cooling rate after annealing is disclosed in Japanese Patent No. 45689 describes a cold rolled steel sheet having improved bake hardenability by adding Ti and Nb in combination. Also, Japanese Patent Application Laid-Open No. H5-25549 proposes a method for producing a cold-rolled steel sheet having improved bake hardenability by adding W, Cr and Mo individually or in combination.
[0006]
In the techniques described in JP-A-55-141526, JP-B-61-45689, and JP-A-5-25549, a coating baking after forming is performed by using solid solution C and solid solution N in steel. Strength is increased by processing. For this reason, there is a problem that the yield strength of the material can be increased but cannot be increased in the tensile strength. Therefore, in the steel plates manufactured by these conventional techniques, the deformation starting stress of the component can be increased, but the effect of increasing the stress required for the deformation over the entire deformation range from the start of the deformation to the end of the deformation of the component is not sufficient. .
[0007]
As a cold-rolled steel sheet capable of increasing the stress required for deformation over the entire deformation range of a part, for example, Japanese Patent Application Laid-Open No. H10-310847 discloses that after forming, heat treatment is performed in a temperature range of 200 ° C. to 450 ° C. There has been proposed an alloyed hot-dip galvanized steel sheet whose tensile strength increases by 60 MPa or more before and after. A steel sheet manufactured by the technique described in JP-A-10-310847 contains C: 0.01 to 0.08%, Mn: 0.01 to 3.0%, and contains W, Cr, and Mo. One or more kinds are contained in a total amount of 0.05 to 3.0%, and if necessary, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, V: A steel sheet having a composition containing one or more of 0.005 to 0.1% and one or more of ferrite and a ferrite-based microstructure, and is heat-treated at a temperature of 220 to 370 ° C. after processing. It is said that fine carbides are formed in the steel, and the strength (tensile strength) after processing is significantly increased. However, in the technique described in Japanese Patent Application Laid-Open No. H10-310847, there is a disadvantage that the heat treatment after processing needs to be performed in a temperature range of 220 to 370 ° C., which is higher than a general baking coating temperature. .
[0008]
Although it is a hot-rolled steel sheet, Japanese Patent Publication No. 23048/1996 discloses that a hot-rolled steel sheet is soft at the time of processing and has a tensile strength of 100 MPa or more as well as yield stress by baking coating at about 170 ° C. after processing. A method for producing a rolled steel sheet is described. In the technique described in Japanese Patent Publication No. 8-23048, steel containing a large amount of C in an amount of 0.02 to 0.13% and N in an amount of 0.0080 to 0.0250% is reheated to 1100 ° C. or more and 850 ° C. Hot rolling is performed to finish the finish rolling at ９００900 ° C., then cooled to a temperature of less than 150 ° C. at a cooling rate of 15 ° C./s or more and wound up, leaving a large amount of solid solution N in the steel, The structure is a composite structure mainly composed of ferrite and martensite.
[0009]
However, using the hot-rolled steel sheet obtained by the technique described in Japanese Patent Publication No. 8-23048 as a starting material, cold rolling and recrystallization annealing are performed, and the cold-rolled steel sheet is not necessarily formed in the same manner as the hot-rolled steel sheet. It is hard to say that an increase in the BH amount or the tensile strength after the heat treatment can be obtained. This is because cold rolling and recrystallization annealing result in a microstructure different from that at the time of hot rolling, and large strain accumulation occurs in cold rolling, so that carbides, nitrides, or carbonitrides are likely to be formed. This is because the state of dissolved N changes.
[0010]
To cope with such a problem, Japanese Patent Application Laid-Open No. 2002-53935 discloses that Al: 0.02% or less, N: 0.0050 to 0.0250%, N / Al is 0.3 or more, and High tension cold rolling with a composition containing 0.0010 or more N in the state and a structure containing 50% or more in terms of area ratio of a ferrite phase having an average crystal grain size of 10 μm or less, with a tensile strength of 440 MPa or more and excellent strain aging hardening characteristics. Steel sheets have been proposed. In the technique described in Japanese Patent Application Laid-Open No. 2002-53935, the BH amount is 80 MPa or more even under the aging treatment condition of holding at 170 ° C. for 20 minutes after the pre-deformation of 5% tensile strain by effectively utilizing the solute N. It is stated that high strain aging hardening characteristics in which the increase in tensile strength before and after the strain aging treatment is 40 MPa or more can be obtained.
[0011]
[Problems to be solved by the invention]
However, recently, the forming conditions have become more severe, and a high-strength cold-rolled steel sheet manufactured by the technique described in Japanese Patent Application Laid-Open No. 2002-53935 has a property that can always sufficiently satisfy these conditions. Nevertheless, there is a demand for further improvement in formability. In particular, recently, a high-tensile cold-rolled steel sheet having high stretch flangeability, high strength-ductility (elongation) balance, and high strain aging hardening characteristics at the same time has been desired.
[0012]
The present invention advantageously solves the above-mentioned problems of the prior art, has a tensile strength of 440 MPa or more, stretch-flangeability, strength-ductility balance and excellent strain-age hardening characteristics, a composite structure type high-tensile cold-rolled steel sheet and The purpose is to propose a manufacturing method thereof.
[0013]
[Means for Solving the Problems]
Heretofore, it has been difficult to obtain a high-tensile cold-rolled steel sheet having simultaneously excellent stretch flangeability, strength-ductility balance, and strain aging hardening characteristics. The present inventors first considered the following in order to achieve the above object.
(1) In a composite structure composed of a ferrite phase and a martensite phase, stress is generated around the martensite phase, which is harder than the ferrite phase, during deformation, which is a starting point of cracks, and the stretch flangeability decreases. In order to reduce the difference in hardness between the ferrite phase and the martensite phase, quenching after annealing and subsequent tempering soften the martensite phase (to form an appropriate amount of tempered martensite phase), By optimizing the heat history at the time, it is effective from the viewpoint of improving the stretch flangeability to distribute the solid solution strengthening elements such as C, N, Si, and Mn in an appropriate amount to the ferrite phase and to harden the ferrite phase. Conceivable.
(2) In order to improve the strength-ductility balance, it is generally considered effective to increase the ferrite phase and refine the martensite phase by using a composite structure of ferrite and martensite. When increasing the strength, if the phase fraction of the martensite phase is excessively increased, the stretch flangeability decreases. In order to simultaneously improve the strength-ductility balance and the stretch flangeability, the phase fraction of the martensite phase is adjusted to a range that does not cause significant deterioration of the strength-ductility balance and the stretch flangeability, and the martensite is softened by tempering. It seems that it is effective to make it more effective. Also,
(3) In order to improve the strain aging characteristics, the amount of solute N in the ferrite phase, which is expected to contribute to the strain aging hardening characteristics by being soft and having more dislocations, may be increased. it is conceivable that. In order to increase the amount of solute N, it is desirable to cool as fast as possible after annealing from the viewpoint of suppressing the precipitation of N as precipitates during cooling after annealing. However, when excessively dissolved N is contained in the ferrite phase, there is a possibility that the ductility of the ferrite is significantly deteriorated and the strength-ductility balance is significantly reduced. In addition, increasing the cooling rate after annealing may significantly increase the martensite phase and decrease ductility, and may degrade aging resistance at room temperature. Therefore, in order to simultaneously improve the stretch flangeability, the strength-ductility balance, and the strain age hardening characteristics, it is considered necessary to optimize the steel composition, the annealing temperature, and the heat pattern during tempering.
[0014]
Based on this idea, more specifically,
(1) Optimizing the contents of C, Mn, and Si, which greatly change the phase fraction and hardness of the ferrite phase and the martensite phase and greatly affect the stretch flangeability and the strength-ductility balance;
{Circle around (2)} Optimizing the amount of N, which effectively works to improve the strain age hardening characteristics, and has a large effect on the hardness of the ferrite phase and the martensite phase; and
(3) Optimization of quenching and tempering conditions after annealing, which affect the refinement and softening of martensite
Various investigations were carried out.
[0015]
Next, the results of basic experiments performed by the present inventors will be described.
In terms of mass%, C: 0.031%, P: 0.011%, S: 0.002%, Al: 0.010%, N: 0.0154%, and the basic composition, Si, Mn, Si: After heating a sheet bar having a composition varied in the range of 0.022 to 1.38% and Mn: 0.15 to 1.95% to 1250 ° C. and soaking, the finish rolling end temperature is 900. A finish rolling consisting of three passes was performed at a temperature of 400 ° C. to obtain a hot-rolled sheet having a thickness of 4.0 mm. After finishing rolling, the hot rolled sheet was kept warm (600 ° C. × 1 h) corresponding to coil winding processing.
[0016]
Next, these hot-rolled sheets were subjected to cold rolling at a rolling reduction of 70% to obtain cold-rolled sheets having a sheet thickness of 1.2 mm. The obtained cold-rolled sheet is annealed at 850 ° C. for 40 seconds, then cooled at an average cooling rate of up to 700 ° C. at a rate of 30 ° C./s, and further cooled from 700 ° C. to 100 ° C. or less. Water cooling was performed at a rate of about 600 ° C./s. Thereafter, tempering was performed at 250 ° C. for 500 seconds to obtain a cold-rolled steel sheet having a composite structure including ferrite, martensite, and tempered martensite.
[0017]
The obtained cold-rolled steel sheet was subjected to a tensile test, a hole expansion test, and a strain age hardening test.
The tensile test was performed in accordance with JIS Z 2241 using a JIS No. 5 tensile test piece with the major axis perpendicular to the rolling direction, and the tensile properties (yield strength YS, tensile strength TS, elongation El). I asked.
[0018]
The hole expansion test uses a test piece having a size of 80 mm x 80 mm x a plate thickness of 1.2 mm in accordance with the provisions of the Japan Iron and Steel Federation Standard JFS T 1001, an initial hole diameter of 10 mm, a die inner diameter of 10.3 mm, The clearance was performed under the condition of 12.5% of the plate thickness, and the hole expansion ratio λ was determined. The hole expansion rate λ is λ (%) = (D _r -D ₀ ) / D ₀ × 100 (where D _r : Hole diameter after breaking (mm), D ₀ : Initial hole diameter (mm)).
[0019]
In the strain age hardening test, a pre-deformation was performed to a tensile pre-strain of 5% using a JIS No. 5 tensile test specimen with the major axis in a direction perpendicular to the rolling direction, and a 170 ° C. × 20 min. After the heat treatment, a tensile test is performed, and the pre-deformation-yield stress YS after the heat treatment is performed. _BH , Tensile strength TS _BH And BH amount = YS _BH -S _5% , ΔTS = TS _BH -TS was calculated. Note that S _5% Is the deformation stress when pre-deformed by 5%, and TS is the tensile strength of the cold rolled steel sheet.
[0020]
The obtained results are shown in FIGS. 1, 2 and 3.
1 shows the relationship between the strength-ductility balance TS × El and {12 (C + N) + Mn-Si}, FIG. 2 shows the relationship between the hole expansion ratio λ and {12 (C + N) + Mn-Si}, and FIG. 3 shows the BH amount. , ΔTS and {12 (C + N) + Mn-Si}. In addition, C, N, Mn, and Si are contents (mass%) of each element.
[0021]
From FIG. 1, FIG. 2, and FIG. 3, by adjusting {12 (C + N) + Mn-Si} in the range of 0.1 to 1.5, the hole expansion ratio λ, the strength-ductility balance TS × El, and the BH amount , ΔTS) at the same time, it can be seen that a composite structure type high tension cold-rolled steel sheet excellent in stretch flangeability, strength-ductility balance, and strain age hardening characteristics can be manufactured. It has also been found that it is important to properly control the annealing temperature and the cooling heat pattern after annealing after adjusting {12 (C + N) + Mn-Si} to the range of 0.1 to 1.5. .
[0022]
The present invention has been completed based on the above findings, with further investigations. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.01 to 0.10%, Si: 0.01 to 1.5%, Mn: 0.1 to 2.0%, P: 0.08% or less, S: 0.005% or less, Al: 0.02% or less, N: 0.0050 to 0.0250%, N / Al is 0.3 or more, and N in solid solution state is 0.0030% or more. And further contains C, N, Mn, and Si by the following formula (1)
0.1 ≦ 12 (C + N) + Mn−Si ≦ 1.5 (1) (where C, N, Mn, and Si: the content (% by mass) of each element)
And a balance consisting of Fe and unavoidable impurities, a ferrite phase having an area ratio of 50 to 96%, a martensite phase of 1 to 30%, and a tempered martensite phase of 3% or more. A composite structure type high-tensile cold-rolled steel sheet having excellent stretch flangeability, strength-ductility balance and strain age hardening characteristics, and having a tensile strength of 440 MPa or more, characterized by having a composite structure containing:
(2) The composite structure type high tension according to (1), further comprising, in addition to the above composition, one or two of Cr and Mo in total in an amount of 2.0% or less in mass%. Cold rolled steel sheet.
(3) The composite according to (1) or (2), further comprising, in addition to the above composition, one or two of Cu and Ni in total of 2.0% or less by mass%. Microstructured high strength cold rolled steel sheet.
(4) In any one of (1) to (3), in addition to the above composition, one or more of Nb, Ti, V, and B may be further expressed by the following formula (2) in mass%.
N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
(Here, N, Al, Nb, Ti, V, B: content of each element (% by mass))
A composite structure type high-tensile cold-rolled steel sheet characterized by containing the following.
(5) In mass%, C: 0.01 to 0.10%, Si: 0.01 to 1.5%, Mn: 0.1 to 2.0%, P: 0.08% or less, S: 0.005% or less, Al: 0.02% or less, N: 0.0050 to 0.0250%, N / Al is 0.3 or more, and C, N, Mn, and Si are (1) Expression
0.1 ≦ 12 (C + N) + Mn-Si ≦ 1.5 (1)
(Here, C, N, Mn, Si: content of each element (% by mass))
A steel slab having a composition contained to satisfy the above is heated to a heating temperature of 1000 ° C. or higher, and then rough-rolled into a sheet bar, and the sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or higher. Hot rolling at a winding temperature of 750 ° C. or lower to form a hot rolled sheet; pickling and cold rolling of the hot rolled sheet to form a cold rolled sheet; (Ac ₁ Transformation point + 20 ° C)-(Ac ₃ (Transformation point + 50 ° C.) After performing an annealing treatment for heating to an annealing temperature in a temperature range of 10 to 120 s, and then changing the annealing temperature to (Ac ₁ Transformation point + 20 ° C)-(Ac ₁ Cooling is performed at an average cooling rate of 5 to 50 ° C./s to a predetermined temperature in a temperature range of (transformation point −100 ° C.), and an average cooling rate of at least 300 ° C. from 100 ° C. to 100 ° C. / S, followed by an annealing step of tempering for 60 to 800 s in a temperature range of 150 to 450 ° C. for 60 to 800 s in order, characterized by stretch flangeability, strength-ductility balance and strain age hardening. A method for producing a composite structure type high-tensile cold-rolled steel sheet having excellent properties and a tensile strength of 440 MPa or more.
(6) The composite structure type high tensile strength according to (5), further comprising, in addition to the above composition, one or two of Cr and Mo in a total amount of 2.0% or less in mass%. Manufacturing method of cold rolled steel sheet.
(7) The composite according to (5) or (6), further comprising, in addition to the above composition, one or two of Cu and Ni in total of 2.0% or less by mass%. Manufacturing method of microstructured high strength cold rolled steel sheet.
(8) In any one of (5) to (7), in addition to the above composition, one or more of Nb, Ti, V, and B may be represented by the following formula (2) in mass%.
N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
(Here, N, Al, Nb, Ti, V, B: content of each element (% by mass))
A method for producing a composite structure type high-tensile cold-rolled steel sheet, characterized by satisfying the following.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
The high-tensile cold-rolled steel sheet according to the present invention is a composite-structure high-tensile cold-rolled steel sheet having a tensile strength TS of 440 MPa or more and having excellent stretch flangeability, strength-ductility balance, and strain age hardening characteristics.
First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, mass% is simply described as%.
[0024]
C: 0.01 to 0.10%
In the present invention, C is contained in an amount of 0.01% or more from the viewpoint of securing a tensile strength of 440 MPa or more and making the structure a composite structure. On the other hand, when the content exceeds 0.10%, the ductility and the formability of the steel sheet are significantly deteriorated due to an increase in the fraction of carbides in the steel. More importantly, when C is contained in an amount exceeding 0.10%, the weldability such as spot weldability and arc weldability is significantly reduced. Therefore, in the present invention, C is limited to the range of 0.01 to 0.10%. In addition, from the viewpoint of further improving the moldability, the content is preferably set to 0.08% or less. Further, for applications requiring particularly good ductility, C is more preferably 0.05% or less.
[0025]
Si: 0.01 to 1.5%
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel. Such an effect is recognized at a content of 0.01% or more. On the other hand, if the content of Si exceeds 1.5%, adverse effects such as impairing the surface properties, chemical conversion treatment properties, and particularly, the aesthetics of the surface. For this reason, Si was limited to the range of 0.01 to 1.5%. Preferably, it is at least 0.1%. For applications requiring a tensile strength exceeding 500 MPa and high ductility, it is desirable to contain 0.5% or more of Si from the viewpoint of a balance between strength and ductility.
[0026]
Mn: 0.1 to 2.0%
Mn has the effect of strengthening steel, and also reduces the critical cooling rate at which a composite structure of ferrite and martensite is obtained, thereby promoting the formation of a composite structure of ferrite and martensite, and refining crystal grains. Has an action. In order to obtain such an effect, in the present invention, it is contained in an amount of 0.1% or more according to the cooling rate after annealing. Mn also has the effect of preventing hot cracking due to S. Further, when high strength of TS500 MPa or more is required, it is desirable to contain 0.5% or more, more preferably 1.0% or more. By increasing the Mn content to this level, there is a great advantage that the variation of the mechanical properties of the steel sheet due to the variation of the manufacturing conditions including the hot rolling conditions, particularly the variation of the strain aging hardening characteristics, is significantly reduced. However, when Mn is excessively contained in excess of 2.0%, although the detailed mechanism is unknown, the hot deformation resistance of the steel sheet tends to increase, and the weldability and the formability of the welded portion further deteriorate. In addition, since the formation of ferrite is suppressed, ductility tends to be significantly reduced. Therefore, the upper limit of Mn is 2.0%. In applications where better corrosion resistance and moldability are required, Mn is desirably 0.80% or less.
[0027]
P: 0.08% or less
P has the effect of strengthening steel, and is preferably contained at 0.001% or more according to the desired strength. However, if it exceeds 0.08%, press formability deteriorates. Therefore, P is limited to 0.08% or less. When more excellent press formability is required, P is preferably set to 0.05% or less. Further, when a large amount of C, Mn or the like is contained in an application requiring a high strength of 590 MPa or more, it is desirable that P be 0.05% or less from the viewpoint of weldability.
[0028]
S: 0.005% or less
S is an element that exists as an inclusion in the steel sheet and deteriorates the ductility, formability, and particularly stretch flangeability of the steel sheet, and is preferably reduced as much as possible in the present invention. When S is reduced to 0.005% or less, the adverse effect on stretch flange formability is negligible. Therefore, in the present invention, the upper limit of S is 0.005%. In the case where more excellent stretch flange formability is required, or in the case where a large amount of C, Mn, etc. is contained in order to secure a tensile strength TS: 590 MPa or more, and excellent weldability is required, Is preferably not more than 0.003%.
[0029]
Al: 0.02% or less
Al is an element that acts as a deoxidizing agent and is useful for improving cleanliness, and is an element that has a function of making the structure finer and is desirably contained at 0.001% or more. In the present invention, N in a solid solution state is also used as a strengthening element. However, aluminum-killed steel to which Al is added in an appropriate range has better mechanical properties than conventional rimmed steel to which Al is not added. I have. On the other hand, when the Al content exceeds 0.02%, an excessive content leads to deterioration of surface properties and remarkable decrease of solid solution N, and it is possible to secure an extremely large strain age hardening characteristic which is an object of the present invention. It will be difficult. Therefore, in the present invention, the upper limit of Al is 0.02%, which is lower than that of conventional steel. From the viewpoint of material stability, Al is desirably in the range of 0.001 to 0.015%. In addition, although there is a concern that a reduction in the Al content may lead to coarsening of the crystal grains, the present invention prevents the reduction by limiting the other alloying elements to optimal amounts and setting the annealing conditions in an optimal range.
[0030]
N: 0.0050 to 0.0250%
N is an element that increases the strength of the steel sheet by the effect of solid solution strengthening and strain aging, and N also has the effect of lowering the transformation point of the steel. This is effective for stabilizing operations in situations where it is not desirable to perform In the present invention, by controlling the manufacturing conditions by containing an appropriate amount of N, a necessary and sufficient amount of N in the solid solution state in the state of the cold-rolled product is ensured, thereby strengthening the solid solution and strain-aging hardening. (Yield strength YS and tensile strength TS) increase effect is sufficiently obtained, and the target tensile strength of 440 MPa or more, and the deformation stress increase (BH amount) of 100 MPa or more before and after the strain aging treatment , And a tensile strength increase (ΔTS) of 60 MPa or more can be stably obtained.
[0031]
The above-mentioned effects such as an increase in strength can be stably obtained by containing N at about 0.0050% or more. On the other hand, if the content exceeds 0.0250%, the incidence of internal defects in the steel sheet increases, and the occurrence of slab cracks and the like during continuous casting becomes significant. For this reason, N was limited to the range of 0.0050 to 0.0250%. From the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process, it is preferable that N is in the range of 0.0070 to 0.0170%. If the N content is within the range of the present invention, there is no adverse effect on weldability.
[0032]
N in solid solution state: 0.0030% or more
In order for a steel sheet to have sufficient strength and to exhibit strain aging hardening due to N effectively, N in the solid solution state (also referred to as solid solution N) must be present in the steel sheet in an amount of approximately 0.0030% or more. is there.
Note that the solid solution N amount is a value obtained by subtracting the precipitated N amount from the total N amount in the steel. In the present invention, the amount of precipitated N is determined by an analytical method to which a dissolution method by electrolytic extraction is applied. Various methods were analyzed as a method for analyzing the amount of precipitated N, but a method of applying a dissolution method by electrolytic extraction using a potentiostatic electrolysis method showed a good correspondence with the actual material change. Note that it is preferable to use an acetylacetone-based electrolyte. The residue extracted by the dissolution method by electrolytic extraction using the constant potential electrolysis method was subjected to chemical analysis to determine the amount of N in the residue, which was defined as the amount of precipitated N.
[0033]
When an increase in the yield stress and an increase in the tensile strength TS due to a larger strain age hardening are required, the amount of solute N is preferably 0.0050% or more.
N / Al ratio: 0.3 or more
In order to stably maintain the dissolved N at 0.0030% or more, it is necessary to limit the content of Al which is an element having an effect of fixing N to strengthening. As a result of investigating the relationship between N remaining in a solid solution state in a cold-rolled steel sheet and the N / Al ratio for a steel sheet in which the combination of components was changed widely, the value of N / Al was found in the range of the steel composition of the present invention steel. Is set to 0.3 or more, it was confirmed that the amount of solid solution N can be stably increased to 0.0030% or more, and a target strain age hardening property can be obtained. Therefore, N / Al is set to 0.3 or more.
[0034]
In the present invention, C, N, Mn, and Si are represented by the following formula (1).
0.1 ≦ 12 (C + N) + Mn-Si ≦ 1.5 (1)
(Here, C, N, Mn, Si: content of each element (% by mass))
Is contained so as to be satisfied. Thereby, as shown in FIG. 1, FIG. 2, and FIG. 3, the stretch flangeability, the strength-ductility balance, and the strain age hardening property can be all improved. That is, in the present invention, it is important to strictly control {12 (C + N) + Mn-Si}. Note that {12 (C + N) + Mn-Si} is preferably 0.1 or more and 1.3 or less.
[0035]
Although there are many unclear points about the detailed mechanism at present, the present inventors consider as follows.
Si is considered to reduce solid solution C and solid solution N in ferrite to make the ferrite phase softer, and improves ductility. However, an excessive content of Si sharply reduces strain aging characteristics. This is considered to be due to a decrease in solid solution N due to precipitation of a compound of Si and N. Contrary to Si, when the total amount of C, N, and Mn exceeds a certain range, the ferrite phase is excessively hardened, so that the strength-ductility balance is rapidly lowered. In the present invention, as a preferable manufacturing method, high-speed cooling is performed after annealing as described later. However, such high-speed cooling tends to provide high stretch flangeability and high strain aging characteristics, but tends to deteriorate ductility. To prevent this deterioration in ductility, the addition of Si, which is a ferrite stabilizing element, is effective.
[0036]
In the present invention, in addition to the above components, one or two of Cr and Mo, one or two of Cu and Ni, and one or two of Nb, Ti, V, and B The above can be appropriately contained.
One or two of Cr and Mo: 2.0% or less in total
Cr and Mo are elements that promote the formation of a martensitic phase, promote the formation of a composite structure, further contribute to uniformity and refinement of crystal grains, and also have an effect of increasing strength. One or two kinds. However, if the content exceeds 2.0% alone or in total, the hot deformation resistance is increased, the chemical conversion property and the surface treatment properties in a broader sense are deteriorated. Decreases part formability. Therefore, it is preferable to limit one or two of Cr and Mo to 2.0% or less in total. In order to obtain the above-mentioned effects, it is preferable to contain Cr: 0.01% or more and Mo: 0.01% or more, respectively.
[0037]
One or two of Cu and Ni: 2.0% or less in total
Cu and Ni are elements having an action of strengthening steel, and one or two of them can be selected and contained as necessary. However, when it is contained alone or in excess of 2.0% in total, the strength-ductility balance tends to decrease. Therefore, it is preferable to limit one or two of Cu and Ni to 2.0% or less in total. In order to obtain the above-mentioned effects, it is more preferable to contain Cu: 0.05% or more and Ni: 0.05% or more, respectively.
[0038]
One or more of Nb, Ti, V, and B: N / (Al + Nb + Ti + V +
B) 0.3 or more
B, Nb, Ti, and V all combine with N and have an effect of strengthening the steel by precipitation strengthening, and one or more of them can be selected as necessary. When these elements B, Nb, Ti, and V are contained, it is preferable that the content satisfies the following expression (2).
[0039]
N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
(Here, N, Al, Nb, Ti, V, B: content of each element (% by mass))
If the expression (2) is not satisfied, that is, if N / (Al + Nb + Ti + V + B) is less than 0.3, the strain age hardening characteristic tends to deteriorate. In order to obtain the above-mentioned effects, it is more preferable to contain B: 0.0001% or more, Nb: 0.001% or more, Ti: 0.001% or more, and V: 0.001% or more.
[0040]
In addition, there is no problem even if Ca, Zr, REM, and the like are contained within the range of a normal steel composition in addition to the above-mentioned components.
The balance other than the components described above is Fe and inevitable impurities. Examples of the inevitable impurities include Sb, Sn, Zn, and Co. As for these unavoidable impurity elements, Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, and Co: 0.1% or less are acceptable.
[0041]
Next, the microstructure of the steel sheet of the present invention will be described.
The cold-rolled steel sheet of the present invention has a ferrite phase, which is a main phase having an area ratio of 50 to 96% with respect to the entire structure, and a martensite phase having an area ratio of 1 to 30% with respect to the entire structure, and 3% as a second phase. It has a composite structure containing the above tempered martensite phase.
If the ferrite phase, which is the main phase, has an area ratio of less than 50% with respect to the entire structure, it is difficult to ensure high ductility, and the press formability tends to decrease. Further, in applications where further good ductility is required, the ferrite phase preferably has an area ratio of 70% or more to the entire structure. In order to utilize the advantages of the composite structure, the ferrite phase needs to be 96% or less.
[0042]
If the martensite phase has an area ratio of less than 1% with respect to the whole structure, a high strength-ductility balance TS × El cannot be secured. On the other hand, when the martensite phase exceeds 30% in area ratio, the stretch flangeability and the strength-ductility balance deteriorate. For this reason, the martensite phase is set to 1 to 30% in area ratio with respect to the whole structure. From the viewpoint of obtaining a higher strength-ductility balance, the martensite phase is preferably set to 3% or more in area ratio. The presence of this martensite phase is also important for obtaining excellent aging resistance at normal temperature.
[0043]
In the present invention, from the viewpoint of improving the stretch flangeability, it is necessary that the second phase further contains a tempered martensite phase having an area ratio of 3% or more to the entire structure. Thereby, the difference in hardness between the ferrite phase, which is the main phase, and the second phase is reduced, and the starting point of cracks during hole expansion can be reduced. Here, the tempered martensite phase refers to a structure in which supersaturated solid solution carbon, alloy element, and the like are precipitated as carbides by tempering in the martensite phase. The martensite phase and the tempered martensite phase can be easily distinguished by observing the structure with a scanning electron microscope or the like.
[0044]
In addition, in addition to the main phase and the second phase described above, any one of a pearlite phase, a bainite phase, and a retained austenite phase may be mixed as a sub phase. However, the pearlite phase, bainite phase, and retained austenite phase are used to more effectively exhibit the effect of the martensite phase. It is preferable that the area ratio be limited to 50% or less of the total).
[0045]
Next, a method for producing a cold-rolled steel sheet according to the present invention will be described.
The composition of the steel slab is the same as the above-described steel sheet composition except for the N in the solid solution state, and thus the description is omitted here.
After smelting the molten steel having the above composition by a known smelting method such as a converter or an electric furnace, it is preferable to form a steel slab by a continuous casting method in order to prevent macro segregation of components. A steel slab may be formed by a known casting method such as an ingot rolling method and a thin slab continuous casting method.
[0046]
The obtained steel slab is then turned into a hot rolled sheet by a hot rolling step. In the hot rolling process, the steel slab is cooled to room temperature and then reheated.In addition, without cooling to room temperature, the steel slab is inserted into a heating furnace as a flake and then rolled, or a small amount of heat is retained. Energy saving processes such as direct rolling and direct rolling, in which rolling is performed immediately after the operation, can be applied without any problem. In particular, direct rolling is one of useful techniques for effectively securing solid solution N in a steel sheet.
[0047]
Next, the reasons for limiting the hot rolling conditions will be described.
Slab heating temperature: 1000 ° C or more
The slab is preferably heated to 1000 ° C. or higher from the viewpoint of securing N (solid solution N) in a solid solution state as an initial state. The upper limit of the heating temperature is not particularly limited, but is desirably set to 1280 ° C. or lower in view of an increase in loss accompanying an increase in the weight of oxidation.
[0048]
The heated slab is rough-rolled into a sheet bar having a predetermined thickness, and then the sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or higher to obtain a hot-rolled sheet.
Finishing roll exit side temperature: 800 ° C or more
By setting the finish-rolling exit side temperature at 800 ° C. or higher, a uniform and fine hot-rolled mother plate structure can be obtained. However, when the finish-rolling exit temperature is less than 800 ° C., the structure of the hot-rolled sheet becomes non-uniform, and the non-uniformity of the structure remains without disappearing even after the subsequent cold rolling and annealing. The risk of malfunctioning increases. Further, when the finish-rolling discharge side temperature is low, even if a high winding temperature is used to avoid the retention of the processed structure, the same problem occurs due to the generation of coarse grains, and the remarkable decrease of the solute N is caused. Therefore, it is difficult to secure a target tensile strength of 440 MPa or more. For this reason, it is preferable that the finish-rolling-side temperature be 800 ° C. or higher. In order to further improve the mechanical properties, it is more preferable to set the finish-rolling exit temperature to 820 ° C. or higher. The upper limit of the finish-rolling exit side temperature does not need to be particularly limited. However, if the finish-rolling exit side temperature is excessively high, the occurrence of scale flaws increases.
[0049]
Winding temperature: 750 ° C or less
If the winding temperature after the finish rolling is higher than 750 ° C., it becomes difficult to secure a tensile strength of 440 MPa or more. For this reason, the winding temperature is preferably set to 750 ° C. or lower. The lower limit of the winding temperature is not strictly limited in terms of the material. However, if the winding temperature is lower than 200 ° C., the shape of the steel sheet is remarkably disturbed, and the risk of causing troubles in actual use increases. Also, the uniformity of the material tends to decrease. For this reason, the winding temperature of the hot rolled sheet is more preferably set to 750 ° C or lower and 200 ° C or higher. When higher material uniformity is required, the winding temperature is desirably 300 ° C. or higher.
[0050]
Then, the hot-rolled sheet is subjected to pickling and cold rolling to be subjected to a cold-rolling step of forming a cold-rolled sheet. The pickling is usually performed according to a known method, but if it is in a very thin scale state, it is possible to perform direct cold rolling. The cold rolling is not particularly limited as long as a cold rolled sheet having a desired size and shape can be obtained. In the present invention, it is not necessary to particularly limit the rolling reduction and the like, but from the viewpoint of surface flatness and texture uniformity, 40% or more. It is preferable to set the rolling reduction of
[0051]
Next, the cold-rolled sheet is subjected to an annealing process, followed by an annealing process including rapid cooling and tempering.
The annealing treatment is performed by (Ac ₁ Transformation point + 20 ° C)-(Ac ₃ (Transformation point + 50 ° C.). Generally, when the annealing temperature is in the two-phase region of ferrite + austenite, N is preferentially distributed to the austenite phase thermodynamically, and it becomes difficult for N to be uniformly dispersed in the ferrite phase and the martensite phase. It is believed that age hardening cannot be expected. However, in the present invention, as described later, since rapid cooling is performed after the annealing treatment, the concentration of C and N in the austenite is suppressed during cooling, and the amount of N deposited is small. From (Ac ₁ Within the temperature range of (transformation point + 20 ° C.) or higher, high strain age hardening characteristics can be obtained even when annealing is performed in the two-phase region. On the other hand, if the annealing temperature is (Ac ₃ If the temperature exceeds (transformation point + 50 ° C.), the crystal grains become coarse and the stretch flangeability, strength-ductility balance, strain age hardening characteristics are deteriorated, and the aging resistance at room temperature is lowered. For this reason, the annealing temperature is (Ac ₁ Transformation point + 20 ° C)-(Ac ₃ (Temperature transformation point + 50 ° C). The heating rate to the annealing temperature is preferably 1 ° C./s or more from the viewpoint of preventing C and N from being concentrated in austenite when passing through the two-phase region of ferrite and austenite, and from the viewpoint of ensuring ductility. It is preferable to be 50 ° C./s or less.
[0052]
The annealing treatment is desirably as short as possible from the viewpoint of refining the structure and securing solid solution N. However, it is desirable that the soaking is performed for about 10 seconds or more from the viewpoint of operation stability. In addition, from the viewpoints of ensuring uniform and fine structure and securing the amount of solid solution N, it is desirable that the thickness be approximately 120 s or less. Such a uniform and fine structure greatly increases the area of the grain boundary, which is considered to be a stable site for solid solution N, and is considered to be effective in improving aging resistance at room temperature.
[0053]
After performing the annealing treatment of heating and soaking under the above conditions, the cold rolled sheet is cooled. Cooling after the annealing treatment is important from the viewpoint of controlling the phase fraction of the ferrite phase or the martensite phase, refining the structure, securing solid solution N, and the like. In the present invention, the cooling from the annealing temperature is two-stage cooling. In the first stage of cooling, (Ac ₁ Transformation point + 20 ° C)-(Ac ₁ (Transformation point −100 ° C.). The second stage cooling following the first stage cooling further comprises (Ac ₁ Transformation point + 20 ° C)-(Ac ₁ (Transformation point-100 ° C) from a predetermined temperature in the temperature range up to 100 ° C or less, at least (Ac ₁ Transformation point + 20 ° C)-(Ac ₁ (Transformation point-100 ° C) Cooling is performed at an average cooling rate of 300 ° C / s or more from a predetermined temperature in the temperature range of 100 ° C.
[0054]
In the first stage of cooling, in order to generate an appropriate amount of martensite phase and to obtain an appropriate composite structure, (Ac) ₁ Transformation point + 20 ° C)-(Ac ₁ It is preferable to limit the average cooling rate to a predetermined temperature in the temperature range (transformation point −100 ° C.) to 5 to 50 ° C./s. If the average cooling rate is out of the range of 5 to 50 ° C./s, it is difficult to secure an appropriate amount of martensite phase. Further, the predetermined temperature, which is the cooling end temperature of the first stage cooling, is set to (Ac ₁ If the temperature is higher than (transformation point + 20 ° C.), an appropriate composite structure cannot be obtained, and it is difficult to obtain a high strength-ductility balance. For this reason, the upper limit of the cooling end temperature of the first stage cooling is set to (Ac ₁ (Transformation point + 20 ° C.). Further, the predetermined temperature which is the cooling end temperature of the first stage cooling is (Ac ₁ (Transformation point −100 ° C.), the strain aging hardening property is reduced. For this reason, the lower limit of the cooling end temperature of the first stage cooling is set to (Ac ₁ (Transformation point −100 ° C.). Note that the cooling end temperature of the first stage cooling may be appropriately set within the above-mentioned temperature range. .
[0055]
In the second stage cooling, immediately after the completion of the first stage cooling, to at least 100 ° C., at least the first stage cooling is completed in order to secure solid solution N and to make the form and amount of the martensite phase appropriate. It is preferable to perform rapid cooling at a rate of 300 ° C./s or more at an average cooling rate from the temperature to 100 ° C. If the average cooling rate is less than 300 ° C./s, it is impossible to secure a predetermined amount or more of solid solution N, and it is not possible to secure excellent stretch flangeability after tempering.
[0056]
The tempering is preferably performed at a tempering temperature of 150 to 450 ° C. and a holding time of 60 to 800 s. In the present invention, an appropriate amount of tempered martensite softer than a normal martensite phase is generated by tempering. If the tempering temperature is lower than 150 ° C, it is difficult to form an appropriate amount of a soft martensite phase (tempered martensite phase). If the tempering temperature is higher than 450 ° C, the martensite phase is excessively reduced, and the tensile strength and ductility are reduced. descend. For this reason, the tempering temperature is preferably in the range of 150 to 450 ° C.
[0057]
Further, the holding time of the tempering is preferably 60 s or more in order to exhibit the above-mentioned tempering effect. On the other hand, a long time exceeding 800 s is not preferable because tempering proceeds too much. According to the study of the present inventors, this tempering treatment also has an effect of suppressing aging deterioration (reduction in elongation, etc.) at room temperature, which is likely to occur when cooling after annealing treatment is performed at high speed.
[0058]
In the present invention, after the above-described annealing step, temper rolling with an elongation of 10% or less may be added for shape correction, adjustment of surface roughness, and the like.
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 a tin-plated steel sheet, an enameled steel sheet, and the like, in addition to a galvanized steel sheet (including an alloy-based steel sheet). Moreover, it goes without saying that the cold-rolled steel sheet of the present invention may be subjected to a special treatment after galvanization in order to improve chemical conversion property, weldability, press formability, corrosion resistance and the like.
[0059]
【Example】
Molten steel having the composition shown in Table 1 was smelted in a converter and made into a steel slab by a continuous casting method. Next, these steel slabs were formed into hot-rolled sheets (also referred to as hot-rolled steel strips) having a thickness of 4.0 mm under the hot rolling conditions shown in Table 2. Subsequently, the hot-rolled steel strip was subjected to a cold-rolling step of pickling and cold-rolling at a rolling reduction of 70% to obtain a cold-rolled sheet having a thickness of 1.2 mm (also referred to as a cold-rolled steel strip).
[0060]
Next, these cold-rolled steel strips were subjected to an annealing step in a continuous annealing line under the conditions shown in Table 2. The resulting cold-rolled steel strip was further subjected to temper rolling at an elongation of 0.5%.
Test specimens were obtained from the obtained cold-rolled steel strip, and were subjected to a structure test, a tensile test, a hole expansion test, and a strain aging hardening test to evaluate tensile properties, stretch flangeability, and strain aging hardening properties. The test method was as follows.
(1) Tissue test
A test specimen was collected from the obtained cold-rolled steel strip, and for a cross section (C cross section) orthogonal to the rolling direction, a microstructure was imaged using an optical microscope or a scanning electron microscope, and an image analysis device was used. The types of microstructures such as a ferrite phase as a main phase, a martensite phase as a second phase, a tempered martensite phase, and a subphase were identified, and their microstructure fractions were determined. The martensite phase and the tempered martensite phase were determined by observing the presence or absence of carbide using a scanning electron microscope.
(2) Tensile test
From the obtained cold-rolled steel strip, a JIS No. 5 tensile test piece whose major axis is perpendicular to the rolling direction was sampled, and a tensile test was performed in accordance with JIS Z 2241, and the tensile properties (yield stress (yield stress ( YS), tensile strength (TS), elongation (El), and yield ratio (YR).
(3) Hole expansion test
The hole expansion test uses a test piece having a size of 80 mm x 80 mm x a thickness of 1.2 mm, an initial hole diameter of 10 mm, an inner diameter of the die of 10.3 mm, and a clearance of the thickness in accordance with JFS T 1001. Was performed under the condition of 12.5%, and the hole expansion ratio λ was determined.
(4) Strain aging hardening test
In the strain age hardening test, a pre-deformation was performed to a tensile pre-strain of 5% using a JIS No. 5 tensile test specimen with the major axis in a direction perpendicular to the rolling direction, and a 170 ° C. × 20 min. After the heat treatment, a tensile test is performed, and the pre-deformation-deformation stress YS after the heat treatment is performed. _BH , Tensile strength TS _BH And BH amount = YS _BH -S _5% , ΔTS = TS _BH -TS was calculated. Note that S _5% Is the deformation stress when pre-deformed by 5%, and TS is the tensile strength of the cold rolled steel sheet.
[0061]
In addition, the solid solution N amount used the value which deducted the precipitation N amount measured by the constant potential electrolysis method from the total N amount obtained by the chemical analysis.
Table 3 shows the obtained results.
[0062]
[Table 1]

[0063]
[Table 2]

[0064]
[Table 3]

[0065]
Each of the examples of the present invention has a tensile strength TS of 440 MPa or more, shows a high hole expansion ratio λ, a high strength-ductility balance TS × El, a high BH amount, and a high ΔTS, and has a stretch flangeability, strength- High tensile strength cold rolled steel sheet with excellent ductility balance and strain age hardening characteristics. On the other hand, in Comparative Examples outside the range of the present invention, any of stretch flangeability, strength-ductility balance, and strain age hardening characteristics are low.
[0066]
【The invention's effect】
As described above, according to the present invention, a high-tensile cold-rolled steel sheet having a tensile strength of 440 MPa or more, having excellent stretch flangeability, strength-ductility balance, and excellent strain aging hardening properties at the same time, and suitable as an automobile part. It can be manufactured inexpensively and stably, and has a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between strength-ductility balance TS × El and {12 (C + N) + Mn—Si}.
FIG. 2 is a graph showing a relationship between a hole expansion ratio λ and {12 (C + N) + Mn-Si}.
FIG. 3 is a graph showing the relationship between the BH amount, ΔTS, and {12 (C + N) + Mn-Si}.

Claims (8)

In mass%,
C: 0.01 to 0.10%, Si: 0.01 to 1.5%,
Mn: 0.1 to 2.0%, P: 0.08% or less,
S: 0.005% or less, Al: 0.02% or less,
N: 0.0050 to 0.0250%
, N / Al is 0.3 or more, N in solid solution is contained at 0.0030% or more, and C, N, Mn, and Si are contained so as to satisfy the following formula (1). A composition having a balance of Fe and unavoidable impurities, a ferrite phase having an area ratio of 50 to 96%, a martensite phase of 1 to 30%, and a tempered martensite phase of 3% or more. A composite structure type high-tensile cold-rolled steel sheet having excellent stretch flangeability, strength-ductility balance, and strain age hardening characteristics, and having a tensile strength of 440 MPa or more.
0.1 ≦ 12 (C + N) + Mn−Si ≦ 1.5 (1)
Here, C, N, Mn, Si: content of each element (% by mass) The composite structure type high tension cold rolling according to claim 1, further comprising, in addition to the composition, one or two of Cr and Mo in a total amount of 2.0% or less by mass in total. steel sheet. The composite structure type high tension according to claim 1 or 2, further comprising, in addition to the composition, one or two of Cu and Ni in a mass% of 2.0% or less in total. Cold rolled steel sheet. 4. The composition according to claim 1, further comprising one or more of Nb, Ti, V and B in mass% in addition to the composition so as to satisfy the following formula (2). 5. The composite structure type high-tensile cold-rolled steel sheet according to any one of the above.
N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
Here, N, Al, Nb, Ti, V, B: content of each element (% by mass) C: 0.01 to 0.10% by mass%, Si: 0.01 to 1.5%,
Mn: 0.1 to 2.0%, P: 0.08% or less,
S: 0.005% or less, Al: 0.02% or less,
N: 0.0050 to 0.0250%
And a steel slab having a composition containing N / Al of 0.3 or more and containing C, N, Mn, and Si so as to satisfy the following expression (1) is heated to a temperature of 1000 ° C. or more. After heating, rough rolling is performed to form a sheet bar, and the sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or higher, and hot rolled into a rolled hot rolled sheet at a winding temperature of 750 ° C. or lower. A cold rolling step in which pickling and cold rolling are performed on the hot-rolled sheet to form a cold-rolled sheet; and (Ac ₁ transformation point + 20 ° C.) to (Ac ₃ transformation point + 50 ° C.) After performing an annealing treatment of heating to an annealing temperature in a temperature range and holding for 10 to 120 s, a predetermined temperature in a temperature range of (Ac ₁ transformation point + 20 ° C.) to (Ac ₁ transformation point−100 ° C.) from the annealing temperature. Cooling with an average cooling rate of 5 to 50 ° C./s until 1 An annealing step of performing cooling so that the average cooling rate from at least the predetermined temperature to 100 ° C. is 300 ° C./s or more to 0 ° C. or less, and then tempering for 60 to 800 seconds in a temperature range of 150 to 450 ° C .; A method for producing a composite structure type high tensile cold-rolled steel sheet having excellent stretch flangeability, strength-ductility balance, and strain age hardening characteristics, and having a tensile strength of 440 MPa or more.
0.1 ≦ 12 (C + N) + Mn−Si ≦ 1.5 (1)
Here, C, N, Mn, Si: content of each element (% by mass) The composite structure type high tension cold rolling according to claim 5, further comprising, in addition to the composition, one or two of Cr and Mo in a total amount of 2.0% or less by mass in total. Steel plate manufacturing method. The composite structure type high tension according to claim 5 or 6, further comprising, in addition to the composition, one or two of Cu and Ni in a mass% of 2.0% or less in total. Manufacturing method of cold rolled steel sheet. 8. The composition according to claim 5, further comprising one or more of Nb, Ti, V, and B in mass% so as to satisfy the following formula (2). The method for producing a composite structure type high-tensile cold-rolled steel sheet according to any one of the above.
N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
Here, N, Al, Nb, Ti, V, B: content of each element (% by mass)

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