JP2005008904A - Cold rolled high tensile strength steel sheet and manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold rolled high tensile strength steel sheet having excellent press formability, bake hardening property and natural aging proof property and also to provide its manufacturing method. <P>SOLUTION: The cold rolled high tensile strength steel sheet has a chemical composition consisting of, by mass, >0.01 to <0.03% C, ≤0.5% Si, 0.5 to 2.5% Mn, ≤0.05% P, ≤0.01% S, 0.005 to 0.15% sol.Al, <0.008% N, ≤0.0025% B, 0.02 to 1.5% Mo and the balance Fe with impurities and satisfying the relations given by expressions sol.Al≥9.7×N and B≥1.5×10<SP>-4</SP>×(Mn<SP>2</SP>+1) and also has a structure in which a primary phase is composed of a ferrite phase and a low temperature transformation phase is contained as a secondary phase. Further, the cold rolled high tensile strength steel sheet can contain 0.003 to 0.15% Ti and/or 0.01 to 1.5% Cr. When manufacturing the steel sheet, hot rolling is performed and the resultant steel plate is coiled at 550 to 750°C, pickled and cold rolled, and the resultant steel sheet is annealed at a temperature between the Ac<SB>1</SB>transformation point and a temperature lower than the Ac<SB>3</SB>transformation point. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００６７】
【表２】

【００６８】
【表３】

【００６９】
【発明の効果】
以上詳述したとおり、本発明によれば、プレス成形などの加工に適用できる十分な成形性を有し、かつ、極めて優れた焼付硬化性を示し、さらに、耐常温時効性に優れた高張力鋼板が製造可能である。したがって、本発明は自動車の車体軽量化を通じて地球環境問題の解決に寄与できるなど産業の発展に寄与するところ大であり、その意義は大きい。
【図面の簡単な説明】
【図１】ＢＨ_０ とＢ含有量とＭｎ含有量の関係を示すグラフである。
【図２】平均ｒ値とｓｏｌ．Ａｌ含有量／Ｎ含有量との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-tensile cold-rolled steel sheet that is used by being formed into various shapes by press working or the like, and a manufacturing method thereof, in particular, a high-tensile cold-rolled steel sheet having good bake hardenability, room temperature aging resistance, and formability. It relates to the manufacturing method.
[0002]
[Prior art]
Now that the industrial technology field is highly divided, materials used in each technical field are required to have special and high performance. High-strength cold-rolled steel sheets used by press forming are also required, and application of high-tensile cold-rolled steel sheets is being studied. In particular, regarding automotive steel sheets, the demand for thin-walled high-tensile cold-rolled steel sheets has been remarkably increasing in order to reduce the weight of the vehicle body and improve fuel efficiency in consideration of the global environment.
[0003]
For example, an automobile outer panel needs to have dent resistance, that is, a property that does not cause permanent deformation when pressed by a finger or hits a stone. The dent resistance increases as the yield stress after press forming and paint baking is higher, and as the plate thickness of the steel plate is thicker. Therefore, if a steel plate having a high yield stress can be used, the thickness can be reduced.
[0004]
On the other hand, in press forming, the thinner the steel sheet used, the easier it is to generate cracks and wrinkles. Therefore, an excellent deep drawability is required. Value) is required. In addition, it is necessary to be well-familiar with the press die and to generate less springback when the molded product is removed from the press die, that is, to have good shape freezing property, and low yield stress before press forming. Is required.
[0005]
Therefore, a steel plate having a high r value and a low yield stress as properties before press forming and having a high yield stress after press forming and paint baking is suitable as a steel plate for automobiles.
[0006]
Even in the prior art, there is a bake hardenable steel plate (BH steel plate) as a steel plate developed to satisfy these characteristics. This is a steel sheet incorporating a so-called strain age hardening phenomenon in which solute C and N atoms segregate on dislocations to fix the dislocations and increase the yield stress. In the process of using the BH steel sheet, dislocations introduced at the time of press forming are fixed by solid solutions C and N at the time of coating baking, and the yield stress increases.
[0007]
Many proposals have been made regarding BH steel sheets. For example, Patent Document 1 and Patent Document 2 describe the production of BH steel sheets with excellent deep drawability, in which Ti and Nb are added to ultra-low carbon steel and Si, Mn, and P are added to increase tensile strength. A method is disclosed. However, this method has the following problems.
[0008]
(1) When a solid solution strengthening element such as Si, Mn, or P is added to increase the tensile strength, not only the tensile strength but also the yield stress increases. As a result, the shape freezing property deteriorates and surface distortion is likely to occur. In addition, it causes deterioration of deep drawability, which is particularly remarkable when Mn is added.
[0009]
(2) When manufacturing a hot dip galvanized steel sheet, non-plating due to addition of Si, deterioration of alloying processability due to addition of P, and the like occur.
(3) It is difficult to achieve both bake hardenability and room temperature aging resistance, and the upper limit of the bake hardening amount is substantially about 50 MPa because of the need to ensure room temperature non-aging.
[0010]
Patent Document 3, Patent Document 4, and Patent Document 5 disclose a method for producing a low-carbon Al-killed steel sheet having a composite structure in which martensite is dispersed in ferrite. A steel sheet having a composite structure is characterized by high tensile strength, low yield stress, and the ability to ensure non-aging at room temperature even when the bake hardening amount is large. However, since the average r value is at most about 1.0 and the deep drawability is inferior, it is difficult to apply to an automobile outer panel.
[0011]
On the other hand, in Patent Document 6, Nb and B are added to an ultra-low carbon steel, and the microstructure is a composite structure of acicular ferrite and ferrite, which is excellent in bake hardenability and room temperature non-aging properties. A method for producing a rolled steel sheet is disclosed. Although a high r value can be obtained by using an extremely low carbon steel as a base, the tensile strength is as low as 380 MPa at most, and it is difficult to increase the strength further.
[0012]
[Patent Document 1] JP 59-31827 A
[Patent Document 2] Japanese Patent Laid-Open No. 59-38337
[Patent Document 3] Japanese Patent Laid-Open No. 55-50455
[Patent Document 4] Japanese Patent Laid-Open No. 56-90926
[Patent Document 5] Japanese Patent Laid-Open No. 56-146826
[Patent Document 6] JP-A-2-232316
[0013]
[Problems to be solved by the invention]
Regarding the problems of these prior arts, according to the results of the study by the present inventors, it has been found that if the C content is low, sufficient bake hardenability may not be obtained even if the microstructure is made into a composite structure. did. The bake hardenability is usually evaluated by applying a tensile pre-strain of 2% and applying a heat treatment corresponding to a paint baking process at about 170 ° C. for about 20 minutes, followed by a tensile test to determine the yield stress after heat treatment. It is said that the difference in 2% deformation stress is obtained and used as the bake hardening amount, and the bake hardening amount is improved as the bake hardening amount is larger. However, in steel sheets with a C content of about 0.03% or less, the bake hardening amount decreases with a decrease in the prestrain amount, and when the prestrain amount is about 0.5% or less, the bake hardenability almost disappears. It will be done.
[0014]
By the way, in recent years, the design of automobiles has been diversified, and the amount of strain generated during press forming varies depending on the part and is distributed in the range of about 0% to 10%. However, a steel plate with a low C content has a small amount of strain. The amount of bake-hardening is lowered at the location, and as a result, the problem that dent resistance cannot be ensured occurs.
[0015]
Here, the present invention has been made in order to know the above-mentioned problems in the prior art and to solve the problems, and the problem is that the press moldability is good and the excellent bake hardenability. It is to provide a high-tensile cold-rolled steel sheet having room temperature aging resistance and a method for producing the same.
[0016]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the inventor conducted a detailed investigation on the influence of additive elements on the tensile properties, bake hardenability, and normal temperature aging resistance of a composite structure steel plate.
[0017]
The test steel was, by mass%, C: less than 0.03%, Si: 0.01%, Mn: 2.5% or less, P: 0.005%, S: 0.005%, sol. Al: 0.005 to 0.1%, N: 0.003%, Mo: 0.3% or less, B: 0.002% or less, balance Fe and chemical composition consisting of inevitable impurities.
[0018]
A steel slab having such a chemical composition is heated to 1240 ° C., then hot-rolled in a temperature range of 900 ° C. or higher, wound at 500 to 650 ° C., and the resulting hot-rolled steel plate is pickled. Cold rolling at a rolling rate of% was followed by continuous annealing.
[0019]
In addition, the virtual difference of a chemical composition was not recognized by the steel piece and the steel plate.
The obtained annealed plate was examined for tensile properties, bake hardenability, and normal temperature aging resistance. Here, the difference between the yield stress of an annealed sheet that was heat-treated at 170 ° C. for 20 minutes without applying a tensile pre-strain and the yield strength of an as-manufactured sheet that was not subjected to such a heat treatment is expressed as BH.₀Defined. As a result of such a preliminary test, the following (A) to (F) were found.
[0020]
(A) BH₀In order to make the steel 30 MPa or more, the microstructure of the steel needs to be a composite structure of a ferrite phase and a low temperature transformation generation phase. This is because when a low-temperature transformation generation phase such as a ferrite phase and a martensite phase is mixed, dislocations are introduced into the ferrite, and solid solution C in the steel is segregated to the dislocations by heat treatment, and the dislocations are fixed. This is probably because the yield stress increases without applying strain.
[0021]
(B) In order to stably obtain a composite structure of a ferrite phase and a low-temperature transformation generation phase in a steel sheet having a C content of less than 0.03%, it is necessary to contain B and Mn.
(C) FIG. 1 shows BH₀And a graph showing the relationship between the B content and the Mn content. ○ in the drawing is BH₀Is more than 30MPa, ● mark is BH₀Is less than 30 MPa.
[0022]
As shown in the figure, BH₀Has a correlation with B content and Mn content,₀In order to increase the Bn content, it is necessary to increase the B content as the Mn content increases.₀It can be seen that
[0023]
B ≧ 1.5 × 10^-4× (Mn²+1) (2)
The reason for this is not clear, but (a) BH₀Increases as the amount of segregation of C atoms to dislocations existing in the annealed plate increases. (B) Attraction interaction acts between C atoms and Mn atoms, and as the Mn content increases, The activity decreases, the amount of deflection to dislocation decreases, and BH₀(C) Since B weakens the interaction between C atoms and Mn atoms, as the B content increases, the amount of dislocation segregation of C increases and BH increases.₀Is estimated to increase.
[0024]
(D) BH₀As aging increases, aging deterioration at normal temperature is likely to occur, but by adding Mo, BH₀Even when the temperature exceeds 30 MPa, room temperature non-aging is ensured. The term “non-aging at normal temperature” as used herein means that the yield point elongation of an annealed sheet left at room temperature for 3 months is 0.2% or less.
[0025]
(E) Although r value of a composite structure steel plate raises with the reduction | decrease of C content, on the other hand, when C content is 0.01% or less, the tensile strength of 390 Mpa or more may not be obtained.
(F) FIG. 2 shows the average r value and sol. It is a graph which shows the relationship of Al content / N content. The r value is sol. It can be seen that the Al content increases as the N content increases. The increase in r value is more remarkable as the coiling temperature after hot rolling is higher. This is caused by sol. The higher the Al content and the higher the coiling temperature after hot rolling, the more easily AlN precipitates in the hot rolled sheet and the amount of solid solution N decreases. This is probably because a crystal texture is formed.
[0026]
From the above results, by adding a certain amount or more of B according to the Mn content and further adding Mo, BH₀Can be increased. Further, depending on the N content, sol. It is possible to compensate for the decrease in the r value that accompanies the increase in strength by containing Al and winding it at a high temperature after hot rolling.
[0027]
Here, the present invention has been made on the basis of such knowledge, and the gist thereof is as follows.
(1) By mass%, C: more than 0.01% and less than 0.03%, Si: 0.5% or less, Mn: 0.5 to 2.5%, P: 0.05% or less, S: 0 .01% or less, sol. Al: 0.005 to 0.15%, N: less than 0.008%, B: 0.0025% or less, Mo: 0.02 to 1.5%, and the following formula (1), ( 2) satisfying the relationship given in 2), with the balance being a chemical composition comprising Fe and impurities, the main phase being a ferrite phase, and the second phase having a structure containing a low-temperature transformation generation phase. Tensile cold-rolled steel sheet.
[0028]
sol. Al ≧ 9.7 × N (1)
B ≧ 1.5 × 10^-4× (Mn²+1) (2)
Here, the element symbol in the formula represents the content of each element in steel in mass%.
[0029]
(2) The high-tensile cold-rolled steel sheet according to (1), wherein the chemical composition is mass% and further contains Ti: 0.003 to 0.15%.
(3) The high-strength cold-rolled steel sheet according to (1) or (2), wherein the chemical composition is mass% and further contains Cr: 0.01 to 1.5%.
[0030]
(4) The steel having the chemical composition according to any one of (1) to (3) above was hot-rolled, wound into a coil at 550 to 750 ° C., pickled and cold-rolled. Later, Ac₁Above the transformation point Ac₃A method for producing a high-tensile cold-rolled steel sheet, characterized by annealing at a temperature below the transformation point.
[0031]
Thus, according to the present invention, a high-strength cold-rolled steel sheet having a tensile strength of 390 to 490 MPa, whose yield stress is increased by 30 MPa or more, preferably 50 MPa or more by heat treatment at 170 ° C. for 20 minutes without applying prestrain. And a method of manufacturing the same.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Next, the reasons for limitations such as the microstructure of the high-tensile cold-rolled steel sheet according to the present invention, the chemical composition of the steel components, rolling, and annealing conditions will be described in detail. In addition, in this specification, all content of a steel component is displayed by the mass%.
[0033]
(A) Steel microstructure
The high-tensile steel sheet according to the present invention includes a composite structure in which a low-temperature transformation generation phase is dispersed in a ferrite phase. This is because good bake hardenability can be obtained without applying pre-strain to the steel sheet. Although the kind of low temperature transformation production | generation phase at this time is not specifically limited, In order to reduce the yield stress of a steel plate as much as possible, it is desirable to use a martensite phase.
[0034]
In addition, as a low-temperature transformation production | generation phase, 2 or more types of phases, for example, a martensite phase and a bainite phase may be included.
When the low temperature transformation generation phase becomes the main phase, the deep drawability deteriorates remarkably, so the main phase is the ferrite phase and the second phase is the low temperature transformation generation phase. The volume fraction of the second phase is preferably less than 10%, and more preferably less than 7%.
[0035]
In addition to the ferrite phase and the low-temperature transformation phase, a residual austenite phase may be included, but in order to maintain good room temperature aging resistance, the volume fraction of the residual austenite phase is determined from the volume fraction of the low-temperature transformation phase. It is also preferable to make it smaller and less than 3%.
[0036]
Here, the “low temperature transformation generation phase” refers to a structure produced by low temperature transformation such as martensite phase or bainite phase. Other examples include an acicular ferrite phase.
[0037]
(B) Chemical composition of steel
C:
When the C content is 0.03% or more, the deep drawability of the steel sheet is significantly impaired. On the other hand, if it is 0.01% or less, the desired tensile strength cannot be obtained. Therefore, the range of content was defined as more than 0.01% and less than 0.03%. Desirably, the lower limit is 0.013% and the upper limit is 0.02%.
[0038]
Si :
Si is an element inevitably contained in the steel, but significantly deteriorates the chemical conversion property of the steel sheet. Moreover, when manufacturing a plated steel plate, plating adhesiveness is reduced. Therefore, the content is preferably small. However, since Si has an effect of strengthening the steel sheet, it can be contained up to 0.5% for the purpose of strengthening the steel. Preferably it is 0.1% or less, More preferably, it is 0.02% or less.
[0039]
Mn:
Mn has the effect of improving the hardenability of the steel, and is contained in an amount of 0.5% or more in order to disperse the low-temperature transformation generation phase in the ferrite phase. On the other hand, since the deep drawability deteriorates if excessively contained, the upper limit of the content is set to 2.5%. Further, since Mn deteriorates the bake hardenability, it is limited to a range satisfying the above formula (2) with the B content. Preferably, the lower limit is 0.7% and the upper limit is 1.7%.
[0040]
P:
P is an element inevitably contained in the steel, but segregates at the grain boundaries and deteriorates secondary work brittleness and weldability. Moreover, when manufacturing a plated steel plate, plating adhesiveness is reduced. Therefore, the smaller the content, the better. However, P can reinforce steel at low cost and can reinforce steel without greatly degrading deep drawability. Good. Preferably, the lower limit is 0.01% and the upper limit is 0.035%.
[0041]
S:
S is an impurity inevitably contained in the steel, and segregates at the grain boundaries and embrittles the steel. Therefore, the content is preferably as small as possible, and is determined to be 0.01% or less.
[0042]
sol. Al :
Al is not only used for deoxidizing molten steel, but also has the function of improving the deep drawability of the composite steel sheet, and is sufficiently contained in the present invention. For this purpose, the sol. Al is contained. This formula shows that sol. Of 5 times or more with respect to the chemical equivalent of Al required to make all N in the steel AlN 3. It means that Al is contained. Preferably, it is more than 0.02% and contains 13.5 times or more of the N content and satisfies the following formula (3), more preferably more than 0.02%, and It is to contain 19.3 times or more and satisfy the following formula (4).
[0043]
sol. Al ≧ 13.5 × N (3)
sol. Al ≧ 19.3 × N (4)
In addition, the sol. When the Al content is less than 0.005%, deoxidation is insufficient. When the Al content exceeds 0.15%, the effect is saturated and uneconomical. For this reason, sol. The Al content was determined to be 0.005 to 0.15% and to satisfy the above formula (2).
[0044]
N:
N is an element inevitably contained in steel, and an increase in the content deteriorates deep drawability and normal temperature aging resistance. Therefore, it was determined to be less than 0.008%. A preferred range is less than 0.005%, and a more preferred range is less than 0.004%.
[0045]
B:
B not only improves hardenability but also has an effect of improving bake hardenability, and is an important constituent in the present invention. Compensates for deterioration of bake hardenability due to Mn addition,₀In order to make the pressure 30 MPa or more, it is limited to a range satisfying the above-mentioned formula (2). However, since the deep drawability is deteriorated, the upper limit is made 0.0025%. A preferable range is 0.0020% or less. The lower limit is not particularly defined, but is preferably 0.0002% or more.
[0046]
Mo :
Mo is contained in an amount of 0.02% or more in order to improve normal temperature aging resistance. However, if the content exceeds 1.5%, the effect is saturated and uneconomical, and the chemical conversion property of the steel sheet deteriorates. Therefore, the content range is set to 0.02 to 1.5%. However, Mo is an expensive element, and when considering its economic efficiency, the upper limit is preferably 0.5%. A preferred range is 0.05 to 0.25%, and a more preferred range is 0.05% to 0.15%.
[0047]
Ti :
Ti is optionally contained and may not be contained in particular. However, Ti may be contained in a range of 0.003 to 0.15% in order to further improve normal temperature aging resistance. Since Ti precipitates and fixes N in steel as TiN, aging deterioration due to N is suppressed. In order to acquire this effect, it is preferable to make it contain 0.003% or more. On the other hand, if it exceeds 0.15%, the effect is saturated and uneconomical. Therefore, when adding Ti, the range of the content was defined as 0.003 to 0.15%.
[0048]
Cr :
Cr is not particularly required to be contained, but is preferably contained because it has an effect of improving hardenability. However, since Cr deteriorates the chemical conversion property of the steel sheet, the upper limit is made 1.5%. On the other hand, if the content is less than 0.01%, the effect of improving hardenability cannot be obtained. Therefore, it was limited to the range of 0.01 to 1.5%. A preferable range is 0.05 to 1.0%, and a more preferable range is more than 0.10% and 0.8% or less.
[0049]
The chemical composition of the steel sheet according to the present invention is the balance Fe and impurities. Examples of impurities at this time include Cu, Ni, V, Ca, Sn, and Sb. % Or less, V, Ca, Sn, Sb and the like are allowed within a range of 0.01% or less.
[0050]
(C) Reason for limitation such as annealing conditions
The steel having the above chemical composition is made into a steel slab by melting by an appropriate means, a continuous casting method, or a method of performing ingot rolling after forming a steel ingot. This steel slab is reheated, or in the case of a high-temperature steel slab after continuous casting or decomposition rolling, hot rolling is performed as it is or with auxiliary heating.
[0051]
The conditions for hot rolling are not particularly specified, but from the viewpoint of performing finish rolling in a low temperature range of austenite to refine the hot rolled sheet crystal grains and developing a recrystallized texture preferable for deep drawability during annealing.₃Transformation point ~ Ar₃It is desirable to perform the final reduction in the range of the transformation point + 100 ° C. In addition, in order to perform final reduction in this temperature range, you may heat a rough rolling material between rough rolling and finish rolling. At this time, it is desirable to heat the rear end of the rough rolled material at a higher temperature than the front end so that the temperature variation over the entire length of the rough rolled material at the start of finish rolling is 140 ° C. or less. Thereby, the uniformity within a coil of a product characteristic improves.
[0052]
Rough rolling material can be heated, for example, by installing a solenoid induction heating device between the roughing mill and the finishing mill and controlling the heating temperature rise based on the longitudinal temperature distribution before the induction heating device. is there.
[0053]
After hot rolling, the steel plate is cooled and wound into a coil at 550 ° C. or higher. This is because if the winding is performed at a temperature lower than 550 ° C., the precipitation of AlN 3 becomes insufficient after winding, and the effect of improving the deep drawability of Al becomes insufficient. It is desirable to wind up at 600 ° C. or higher. On the other hand, when the winding temperature exceeds 750 ° C., the yield due to scale generation is reduced, so the upper limit of the winding temperature is set to 750 ° C.
[0054]
Cold rolling is performed according to a conventional method after descaling by pickling or the like. In order to develop a recrystallized texture preferable for deep drawability by recrystallization annealing performed after cold rolling, the rolling reduction is preferably set to 70% or more.
[0055]
Annealing:
The cold-rolled steel sheet is subjected to a treatment such as degreasing according to a known method, if necessary, and is recrystallized and annealed. The annealing temperature at this time is such that the microstructure of the steel is a composite structure in which the main phase is a ferrite phase and the second phase is a low-temperature transformation generation phase.₁Above the transformation point, Ac₃The temperature range is below the transformation point. This is because the annealing temperature is Ac₁If the temperature is less than the transformation point, a low-temperature transformation generation phase cannot be obtained.₃This is because if it is at or above the transformation point, a single-phase structure consisting only of a low-temperature transformation-forming phase is formed, and the bake hardenability and deep drawability are significantly reduced.
[0056]
Where Ac₁The transformation point is α → γ transformation start temperature, Ac₃The transformation point is the α → γ transformation completion temperature. In order to obtain a low-temperature transformation generation phase, the average cooling rate up to 550 ° C. after annealing is preferably 5 ° C./s or more.
[0057]
After annealing, temper rolling may be performed according to a conventional method. However, since the elongation is reduced, the elongation of temper rolling is preferably 1.0% or less. More preferably, the elongation is set to 0.4% or less.
[0058]
The cold-rolled steel sheet produced according to the method of the present invention can be electroplated as a base material or used as a coated steel sheet. Further, the steel sheet after the cold rolling may be annealed in a heating furnace equipped in a known hot dipping apparatus and then hot-dip plated to be a plated steel sheet or an alloyed hot-dip steel sheet. Of course, after annealing in a continuous annealing furnace, hot dipping may be performed to form a plated steel sheet or an alloyed hot dipped steel sheet. However, when the Cr content exceeds 0.1%, uneven alloying of the plating tends to occur, and therefore, it is preferable to use a steel plate other than the alloyed hot-dip plated steel plate.
[0059]
【Example】
Examples of the present invention will be described below.
Steels having chemical compositions shown in Table 1 were melted and cast using a laboratory vacuum melting furnace. These steel ingots were made into steel pieces with a thickness of 30 mm by hot forging, heated to 1240 ° C. using an electric heating furnace, and held for 1 hour. After extracting the steel piece from the heating furnace, hot rolling was started in a temperature range of 900 ° C. or higher using a laboratory hot rolling mill to obtain a hot rolled steel sheet having a thickness of 5 mm.
[0060]
Immediately after hot rolling, it is cooled to 500 to 650 ° C. by forced air cooling or water spray cooling, and this is taken up as a coiling temperature, charged in an electric heating furnace maintained at the same temperature and held for 1 hour, then 20 ° C. Furnace cooling was performed at a cooling rate of / h to obtain a slow cooling treatment after winding.
[0061]
Both surfaces of the obtained steel plate are ground to form a cold rolled base metal having a thickness of 4 mm, cold rolled at a rolling rate of 80%, and held at a soaking temperature of 740 to 860 ° C. shown in Table 2 for 40 seconds. Recrystallization annealing equivalent to continuous annealing was performed. Thereafter, these annealed plates were subjected to temper rolling with an elongation of 0.2%, and the performance was evaluated.
[0062]
The r value is measured by subjecting a JIS No. 5 tensile specimen taken from the rolling direction (0 degree direction), 45 degree direction, and width direction (90 degree direction) to a tensile test, and the average r value is (r₀+ 2 × r₄₅+ R₉₀) Calculated from / 4.
[0063]
Yield stress (YS), tensile strength (TS), yield point elongation (YPE) and total elongation were obtained by conducting a tensile test on a JIS No. 5 tensile specimen taken from the width direction.
The bake hardenability was evaluated by the following method. A JIS No. 5 tensile test piece was taken from the width direction of the cold rolled steel sheet, subjected to a heat treatment at 170 ° C. for 20 minutes, and then subjected to a tensile test. The difference between the YS obtained at this time and the YS obtained by subjecting it to a tensile test without heat treatment was expressed as BH.₀Defined. Further, a 2% tensile pre-strain was applied to a JIS No. 5 tensile test piece collected from the width direction, and subjected to a heat treatment at 170 ° C. for 20 minutes, and then subjected to a tensile test. The difference between the obtained YS and 2% deformation stress is expressed as BH₂These were defined as an index of bake hardenability.
[0064]
For aging resistance at normal temperature, hold a JIS No. 5 tensile test specimen taken from the width direction in an electric furnace set at 40 ° C for 3 months, then subject it to a tensile test and measure the elongation at yield (YPE). It was evaluated by.
[0065]
Table 3 summarizes the results of performance evaluation as described above. All of the test results (test numbers 2, 7, 8, 10, 11, 13, 18, 19) of the cold-rolled steel sheets manufactured under the conditions within the scope of the present invention have an average r value of 1.3 or more. And YS was 240 MPa or less, indicating good press formability. BH₀Is 30 MPa or more, BH₂Was 50 MPa or more, and showed excellent bake hardenability. Further, the YPE after aging treatment at 40 ° C. for 3 months was 0.2% or less, indicating good room temperature aging resistance.
[0066]
[Table 1]

[0067]
[Table 2]

[0068]
[Table 3]

[0069]
【The invention's effect】
As described above in detail, according to the present invention, the present invention has sufficient formability that can be applied to processing such as press molding, exhibits extremely excellent bake hardenability, and has high tensile strength with excellent room temperature aging resistance. Steel sheets can be manufactured. Therefore, the present invention greatly contributes to the development of industries such as being able to contribute to solving global environmental problems through weight reduction of automobile bodies, and its significance is great.
[Brief description of the drawings]
FIG. 1 BH₀It is a graph which shows the relationship between B content and Mn content.
FIG. 2 shows average r value and sol. It is a graph which shows the relationship with Al content / N content.

Claims (4)

In mass%, C: more than 0.01% and less than 0.03%, Si: 0.5% or less, Mn: 0.5 to 2.5%, P: 0.05% or less, S: 0.01% Hereinafter, sol. Al: 0.005 to 0.15%, N: less than 0.008%, B: 0.0025% or less, Mo: 0.02 to 1.5%, and the following formula (1) (2) ) With the chemical composition comprising the balance Fe and impurities, the main phase being a ferrite phase, and the second phase having a structure containing a low-temperature transformation generation phase. Cold rolled steel sheet.
sol. Al ≧ 9.7 × N (1)
B ≧ 1.5 × 10 ⁻⁴ × (Mn ² +1) (2)
Here, the element symbol in the formula represents the content of each element in steel in mass%. 2. The high-tensile cold-rolled steel sheet according to claim 1, wherein the chemical composition is in mass% and further contains Ti: 0.003 to 0.15%. 3. The high-tensile cold-rolled steel sheet according to claim 1, wherein the chemical composition is mass% and further contains Cr: 0.01 to 1.5%. The steel having the chemical composition according to any one of claims 1 to 3 is hot-rolled, wound into a coil at 550 to 750 ° C, pickled and cold-rolled, and then Ac ₁ method for producing a high-strength cold-rolled steel sheet, which comprises annealing at a temperature below the transformation point or above Ac ₃ transformation point.

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