JPH11199973A - Composite structure high-strength cold-rolled steel sheet excellent in fatigue properties and method for producing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a composite structure high-strength cold-rolled steel sheet excellent in fatigue characteristics and a method for producing the same. SOLUTION: C: 0.03 to 0.20%, Cu: 0.
Ferrite containing 2 to 2.0%, B: 2 to 20 ppm
A martensitic composite structure steel sheet, in which the presence state of Cu in the ferrite phase is characterized by the fact that the size of particles composed solely of Cu is 2 nm or less in a solid solution state and / or a precipitation state. An excellent composite structure high-strength cold-rolled steel sheet, and hot rolling of the steel of the above-described components, are performed according to a conventional method except that finish rolling is performed at an Ar ₃ transformation point or higher, and subsequently according to a conventional method, pickling and cold rolling. After the annealing, when performing continuous annealing, after holding for 30 to 150 seconds in a two-phase region of not less than Ac ₁ transformation point and not more than Ac ₃ transformation point, it is cooled to a temperature region of 400 ° C. or less at a cooling rate of 20 ° C./s or more. A method for producing the above steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet having a composite structure excellent in fatigue properties and a method for producing the same. The present invention relates to a composite structure high-strength cold-rolled steel sheet excellent in fatigue characteristics suitable as a material and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, in order to improve fuel efficiency of automobiles,
For the purpose of weight reduction, application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted. Where Al
Light metals such as alloys have the advantage of high specific strength, but are significantly more expensive than steel, so their use has been limited to special applications. In order to reduce the weight of automobiles in a wider range, there is a strong demand for the use of inexpensive high-strength steel sheets.

In general, the higher the strength of a material, the lower the ductility and workability (formability) as well as the notch sensitivity. Therefore, when applying high-strength steel sheets to parts that may cause fatigue failure due to vibrations during running, not only study the formability but also the stress concentration coefficient of the stress concentration parts such as notches and welds. In addition to the reduction considerations, the fatigue durability of the steel sheet itself is also an important consideration.

[0004] As a high-strength cold-rolled steel sheet excellent in workability,
For example, the invention of a high-strength cold-rolled steel sheet having a transformed structure with a low yield ratio and excellent ductility (composite structure) has been disclosed in JP-A-62-74024.
No. 6,009,036. Furthermore, for high-strength cold-rolled steel sheets having this type of ferrite, martensite, or a composite structure containing partially retained austenite, by optimizing the microstructure of the hot-rolled sheet before cold rolling and annealing. The invention to improve the fatigue characteristics is disclosed in
No. 105930 and JP-A-64-79322.

[0005]

However, in some parts of automobile parts that may cause fatigue failure due to vibrations during running, etc., it is necessary to achieve both workability such as elongation, low yield ratio, and fatigue durability. Is very important, and even in the above-mentioned prior art, the compatibility is still insufficient, and a demand for further improvement in characteristics is increasing.

An object of the present invention is to clarify the properties of a steel sheet for achieving both fatigue properties and workability and a method for producing the same, and to provide a composite structure high-strength cold-rolled steel sheet having excellent fatigue properties and a method for producing the same. It is assumed that.

[0007]

SUMMARY OF THE INVENTION The present inventors have developed high-strength steels produced on an industrial scale by continuous hot rolling, followed by pickling, cold rolling, and continuous annealing equipment which are now commonly employed. With the manufacturing process of cold-rolled steel sheets in mind, we conducted intensive research to achieve both fatigue properties and workability of high-strength cold-rolled steel sheets. As a result, Cu or C
The present invention has been found that a Cu precipitate composed of u alone and having a particle size of 2 nm or less is very effective in improving fatigue characteristics and does not impair workability.

Hereinafter, the results of basic research that led to the present invention will be described. First, an investigation was made on the effect of the size of a particle composed of Cu alone in the ferrite phase on fatigue characteristics. The test material for that was prepared as follows. That is, 0.05% C-1.0% Si-1.
4% Mn-1.0% Cu-0.5% Ni-0.0003
% B, the slab was melted and hot-rolled, wound at room temperature, pickled, and then 3.0 to 1.2 mm.
After the cold-rolled steel sheet which had been cold-rolled by 60% to 800 ° C. was maintained at 800 ° C. for 60 seconds, a test material subjected to an annealing treatment of water cooling was prepared. Further, after holding at 100 to 600 ° C. for 1 hour, a heat treatment of furnace cooling is performed, whereby the microstructure has a composite structure including ferrite as a main phase and martensite as a second phase. A steel sheet was obtained in which the size of a single particle was varied. Note that the second phase here is mainly martensite, but may partially contain retained austenite.
FIG. 1 shows the results of a fatigue test performed on these steel sheets. From these results, in the steel sheet having a composite structure including a ferrite phase and a martensite phase and a partly retained austenite phase, there is a strong correlation between the average size of the particles composed solely of Cu in the ferrite phase and the fatigue limit ratio, It has been newly found that the fatigue limit ratio is remarkably improved when the average size of the particles composed of Cu alone in the ferrite phase is 2 nm or less. Further, by restricting the hot rolling conditions, the cold rolling reduction, the annealing conditions, and the like, the average size of the particles composed solely of Cu in the ferrite phase is 2%.
It was also newly found that a steel plate having a thickness of nm or less can be manufactured.

Next, the effect of element B on the fatigue characteristics was investigated. The test material for that was prepared as follows. That is, 0.05% C-1.0% Si
Based on -1.4% Mn-0.5% Ni steel.
To a steel to which 0% Cu has been added and a steel to which Cu has not been added, a slab obtained by adjusting the composition of a steel having a changed B content concentration is hot-rolled and wound at room temperature. After pickling,
A cold-rolled steel sheet that has been cold-rolled by 60% from 3.0 mm to 1.2 mm is kept at 800 ° C. for 60 seconds, and then subjected to an annealing treatment of water cooling, so that the microstructure has a ferrite main phase and a A steel sheet having a composite structure having the site as the second phase was obtained. FIG. 2 shows the results of a fatigue test performed on these steel sheets. The results show that there is a strong correlation between the B content concentration and the fatigue limit ratio only for steel to which 1.0% Cu is added, and that the fatigue limit ratio is significantly improved when the B content concentration is 2 ppm or more. Headlined.

[0010] Regarding the mechanical properties by the tensile test, the test piece No. 5 described in JIS Z 2201 was measured by JI
It was measured by the test method described in SZ2241. Further, the fatigue properties of the steel sheet are as shown in FIG. 3, the thickness is 1.2 mm, the length is 90 mm, the width is 18 mm, the width of the minimum cross section is 10 mm,
A value obtained by dividing the fatigue strength σW at 2 × 10 ⁶ times obtained by the plane bending fatigue test in which the notch has a curvature radius of 30 mm using a plane bending fatigue test piece having a notch radius of curvature of 30 mm by the tensile strength σB of the steel sheet. (Fatigue limit ratio σW / σB).

[0011] For particles composed of Cu alone in the ferrite phase, a transmission electron microscope sample was taken from a quarter of the thickness of the test steel and subjected to energy dispersive X-ray spectroscopy (Energy Dispersive X-ray).
Spectroscope (EDS) or electron energy loss spectroscopy (Electron Energy Loss)
Field emission electron gun (Field Emission Gun: FEG) with an accelerating voltage of 200 kV, which has a composition analysis function of s Spectroscope (EELS).
Observed by a transmission electron microscope equipped with. The composition of the observed particles was determined by EDS and EELS.
u alone. Further, the size of the particles composed solely of Cu in the ferrite phase defined in the present invention is an average value in one field of view of each of the measured particle sizes.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) In mass%, C: 0.03 to 0.20%, Si:
0.1-2.0%, Mn: 0.5-3.0%, P ≦ 0.
02%, S ≦ 0.01%, Al: 0.005 to 0.1
%, Cu: 0.2-2.0%, B: 0.0002-0.
0020%, the balance being Fe and unavoidable impurities, the microstructure of which is a composite structure having ferrite as a main phase and martensite as a second phase, and the state of existence of Cu in the ferrite phase. Is a composite-structure high-strength cold-rolled steel sheet excellent in fatigue characteristics, characterized in that particles composed of Cu alone are in a solid solution state and / or a precipitation state of 2 nm or less.

(2) The steel further comprises N
i: The composite structure high-strength cold-rolled steel sheet according to the above (1), which contains 0.1 to 1.0%. (3) The steel further contains, by mass%, Ca: 0.00
The composite structure having high fatigue strength according to the above (1) or (2), comprising one or two of 5 to 0.02% and REM: 0.005 to 0.2%. Cold rolled steel sheet.

(4) The steel further comprises, by mass%, M
o: 0.05-0.2%, V: 0.02-0.2%, T
i: 0.01 to 0.2%, Nb: 0.01 to 0.1%,
Cr: 0.01-0.3%, Zr: 0.02-0.2%
The high-strength cold-rolled steel sheet with a composite structure excellent in fatigue properties according to any one of the above (1) to (3), comprising one or more of the following.

(5) The hot rolling of the slab having the components described in any of the above (1) to (4) is carried out according to a conventional method except that the finish rolling is carried out at the Ar ₃ transformation point or higher. After performing pickling and cold rolling according to the method, when performing continuous annealing, after holding for 30 to 150 seconds in a two-phase region from the Ac ₁ transformation point to the Ac ₃ transformation point, cooling at 20 ° C./s or more The microstructure is characterized by cooling to a temperature range of 400 ° C. or less at a speed, and the microstructure is a composite structure having ferrite as a main phase and martensite as a second phase, and is composed of Cu alone in the ferrite phase. 2nm particle size
A method for producing a composite structure high-strength cold-rolled steel sheet having excellent fatigue characteristics as described below.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
First, the microstructure of the steel sheet and the existing state of Cu according to the present invention will be described. The microstructure of the steel sheet is a composite structure having ferrite as a main phase and martensite as a second phase in order to ensure excellent workability. However, the second phase permits to partially contain retained austenite. In order to secure good ductility for guaranteeing good workability and a low yield ratio of 70% or less, the volume fraction of ferrite is preferably 50% or more and the volume fraction of retained austenite is preferably less than 5%. .

The presence state of Cu in the ferrite phase is a solid solution state and / or a precipitation state in which the size of the particle composed of Cu alone is 2 nm or less. This makes it possible to improve the fatigue properties while suppressing an increase in static strength that leads to deterioration in workability, that is, without impairing the excellent workability of the composite structure steel sheet of ferrite and martensite. On the other hand, if the size of the particle composed of Cu alone in the ferrite phase is more than 2 nm, Cu
Since the precipitation strengthening of the steel significantly increases the static strength of the steel sheet, the workability is significantly deteriorated. In addition, in such precipitation strengthening of Cu, the fatigue limit does not increase as much as the static strength increases, so that the fatigue limit ratio decreases. Therefore, the size of the particle composed of Cu alone in the ferrite phase needs to be 2 nm or less.

Next, the reasons for limiting the chemical components of the present invention will be described. If the content of C exceeds 0.20%, the workability and the weldability deteriorate, so the content is made 0.20% or less. Further, when the content is less than 0.03%, the volume ratio of martensite in the structure is reduced and the strength is reduced. Si promotes ferrite transformation and raises the C concentration in untransformed austenite to form a composite structure.
It is necessary to add 0.1% or more. On the other hand, if more than 2.0% is added, the scale-off amount increases and the yield decreases, so the upper limit is made 2.0%.

Mn is required to be 0.5% or more in order to obtain the desired second phase, martensite. Also, 3.
If more than 0% is added, slab cracks occur, so the content is made 3.0% or less. If P is added in excess of 0.02%, it not only adversely affects the workability and weldability, but also segregates at the grain boundaries, lowering the grain boundary strength and causing grain boundary embrittlement. .

If S is too large, it causes cracking during hot rolling, so it should be reduced as much as possible, but if it is 0.01% or less, it is in an acceptable range. Al must be added in an amount of 0.005% or more for the deoxidation of molten steel. However, if added in a large amount, not only increases nonmetallic inclusions and deteriorates elongation, but also causes an increase in cost. The upper limit is set to 0.1%.

Cu is one of the most important elements in the present invention, and has the effect of improving the fatigue properties by forming a solid solution or a precipitate having a particle size of 2 nm or less. However,
If it is less than 0.2%, the effect is small, and even if it is added more than 2.0%, the effect is saturated. Therefore, the addition range is limited to 0.2 to 2.0%. B is one of the most important elements in the present invention, and has an effect of increasing the fatigue limit by being combined with Cu. However, if it is less than 0.0002%, it is insufficient to obtain the effect, and 0.0020%
If added excessively, slab cracking occurs. Therefore, the addition of B is set to 0.0002% or more and 0.0020% or less.

Ni is an element that also promotes the formation of ferrite, and is added to prevent hot brittleness due to the inclusion of Cu. However, if it is less than 0.1%, the effect is small,
Even if added in excess of 1.0%, the effect is saturated,
0.1 to 1.0%. Ca and REM are elements that become the starting point of destruction or change the form of nonmetallic inclusions that degrade workability and render them harmless. However, even if each is added less than 0.005%, there is no effect. If Ca is added more than 0.02%, and if REM is added more than 0.2%, the effect is saturated. ~ 0.02
%, REM: 0.005 to 0.2%.

Further, in order to impart strength, Mo,
One or two or more elements of precipitation strengthening or solid solution strengthening of V, Ti, Nb, Cr and Zr may be added. However, Mo: less than 0.05%, V: less than 0.02%, T
i: less than 0.01%, Nb: less than 0.01%, Cr:
If it is less than 0.01% and Zr: less than 0.02%, the effect cannot be obtained. Mo: more than 0.2%, V:
More than 0.2%, Ti: more than 0.2%, Nb: more than 0.1%, C
Even if r: more than 0.3% and Zr: more than 0.2%, the effect is saturated.

Next, the reason for limiting the production method of the present invention will be described in detail below. In the present invention, a slab obtained by casting molten steel whose components are adjusted to the target component content is subjected to high-temperature casting. The pieces may be directly sent to a hot rolling mill, or may be cooled to room temperature and then reheated in a heating furnace for hot rolling. There is no particular limitation on the reheating temperature,
If the temperature is 1350 ° C. or more, the scale-off amount becomes large and the yield decreases, so the reheating temperature is desirably less than 1350 ° C.

The hot rolling step needs to be completed in a temperature range where the final rolling final pass temperature (FT) is equal to or higher than the Ar ₃ point. This is because, if the rolling temperature falls below the Ar ₃ point during hot rolling, strain remains in the ferrite grains and the strength increases, which hinders subsequent cold rolling. Cooling after finish rolling and winding temperature (CT) are not particularly defined because structure control, precipitate control, and the like are performed in the annealing step after cold rolling. In order to make it easier to obtain a ferrite-martensite composite microstructure after annealing, it is desirable that the composition distribution be completed in the hot-rolled sheet stage. Therefore, cooling after finishing rolling is completed at the Ar ₃ transformation point. It is preferable to air-cool for 1 to 10 seconds in the temperature range from to the Ar ₁ transformation point. Regarding the subsequent cooling and winding temperatures, it is desirable to keep Cu in a solid solution state even in the hot-rolled sheet stage in order to bring Cu into a solid solution state during annealing. Below, the cooling rate to the temperature range is preferably 20 ° C./s or more.

The pickling and cold rolling steps after winding may be performed according to a conventional method, and are not particularly defined in the present invention. However, if the rolling reduction of the cold rolling is less than 30%, recrystallization does not completely occur in the subsequent annealing step, and the ductility is likely to be deteriorated. Since a load is applied, the rolling reduction of the cold rolling is preferably 30% or more and 80% or less.

In the present invention, the annealing step is based on continuous annealing. The heating temperature in the continuous annealing is in a two-phase range from the Ac ₁ transformation point to the Ac ₃ transformation point. If the temperature is too low even within the temperature range, if cementite or Cu is precipitated in the hot-rolled sheet stage, it takes too much time for cementite or Cu to re-dissolve in solid solution. The C content in the austenite decreases due to the rate of being too large, and the nose of bainite or pearlite transformation is liable to occur in the subsequent cooling, so that a predetermined microstructure cannot be obtained.
The heating is preferably performed at a temperature of not less than 850 ° C and not less than 850 ° C.

On the other hand, if the holding time at the heating temperature is less than 30 seconds, if cementite or Cu is precipitated in the hot-rolled sheet stage, it is insufficient to completely re-dissolve cementite or Cu. On the other hand, if it exceeds 150 seconds, the sheet passing speed must be reduced, which is not preferable in terms of operation. Therefore, the holding time is set to 30 to 150 seconds. The cooling rate after heating and holding
If the temperature is less than 20 ° C./s, there is a possibility that the nose of bainite or pearlite transformation occurs, and a predetermined microstructure cannot be obtained.

If the cooling end temperature is higher than 400 ° C., bainite is formed and the desired ferrite-martensite composite microstructure cannot be obtained. Further, since the precipitation of Cu is promoted to cause coarse precipitation of Cu and the intended fatigue characteristics cannot be obtained, it is necessary to cool to a temperature range of 400 ° C. or lower.

[0030]

The present invention will be further described below with reference to examples. Steels A to Y having the chemical components shown in Tables 1 and 2 (continued from Table 1) were melted in a converter, continuously cast, reheated at a heating temperature of 1230 ° C., and finished at a temperature of 790 to 790 ° C. 830 ° C
After hot rolling at room temperature to 450 ° C., pickling, and further, at a rolling reduction of 60 to 80%, 0.7 to 1.6 mm
After cold rolling to a sheet thickness of Table 3, Table 3 and Table 5 (continuation of Table 3-
Annealing was performed under the conditions shown in 2). In addition, the indication of the chemical composition in Tables 1 and 2 is% by mass.

The tensile test of the annealed plate thus obtained was performed by first processing the test material into a No. 5 test piece described in JIS Z 2201 and following the test method described in JIS Z 2241. Table 3, Table 4 (continuation of Table 3-1), Table 5
(Continued in Table 3-2) and Table 6 (Continued in Table 3-3) show the test results. Further, a plane bending fatigue test of complete swinging was performed on a plane bending fatigue test piece having a length of 90 mm, a width of 18 mm, a width of a minimum cross section of 10 mm, and a radius of curvature of a notch of 30 mm as shown in FIG. went. The fatigue properties of the steel sheet were evaluated by the value obtained by dividing the fatigue strength σW at 2 × 10 ⁶ times by the tensile strength σB of the steel sheet (fatigue limit ratio σW / σB).

As for the particles composed solely of Cu in the ferrite phase, a transmission electron microscope sample was taken from a quarter of the thickness of the test steel and subjected to energy dispersive X-ray spectroscopy (EDS) and electron energy loss. Observation was made with a transmission electron microscope equipped with a field emission electron gun (FEG) having an accelerating voltage of 200 kV and having a composition analysis function of spectroscopy (EELS). The observed composition of the particles was confirmed to be solely Cu by EDS and EELS. Further, the size of the particles composed solely of Cu in the ferrite phase defined in the present invention is an average value in one field of view of each of the measured particle sizes.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

[Table 6]

According to the present invention, steels A-1, B-1,
C-1, D-1, F-1, H-1, I-1, J-1, K
-1, L-1, N-1, O-1, P-1, Q-1, R-
1, U-1, V-1, X-1, Y-1 19 steels,
A multi-structure high-strength cold-rolled steel sheet having excellent fatigue characteristics in which the size of particles composed solely of Cu in ferrite, which is the main phase, is 2 nm or less has been obtained.

Other steels are outside the scope of the present invention for the following reasons. That is, since the heating temperature (ST) of the steel A-2 is lower than the range of the present invention, re-dissolution of cementite is insufficient, and a desired composite microstructure of ferrite-martensite cannot be obtained after annealing. A low yield ratio (YR) and a sufficient fatigue limit ratio (σW / σB), which are one of the indexes of the properties, are not obtained.

Since the retention time of steel A-3 is shorter than the range of the present invention, re-dissolution of cementite and Cu becomes insufficient,
Sufficient fatigue limit ratio has not been obtained. In steel E-1, since the content of P is larger than the range of the present invention, P segregates at the grain boundary to lower the grain boundary strength, and a sufficient fatigue limit ratio is not obtained. Since steel G-1 has a Cu content lower than the range of the present invention, the effect of improving the fatigue properties is small, and a sufficient fatigue limit ratio has not been obtained.

In steel J-2, the cooling rate after heating is lower than the range of the present invention, so that pearlite and bainite are formed, the desired composite microstructure of ferrite-martensite cannot be obtained, and a low yield ratio is obtained. And a sufficient fatigue limit ratio has not been obtained. Since steel J-3 has a cooling end temperature higher than the range of the present invention, bainite is formed and a desired ferrite-martensite composite microstructure cannot be obtained. Further, since precipitation of Cu is promoted and coarse Cu is deposited, a low yield ratio and a sufficient fatigue limit ratio cannot be obtained.

Since the content of B in the steel M-1 is less than the range of the present invention, the effect of increasing the fatigue limit by the combined addition with Cu is insufficient, so that a sufficient fatigue limit ratio can be obtained. Not. Since the steel S-1 has a Si content lower than the range of the present invention, the effect of accelerating ferrite transformation and increasing the C concentration in untransformed austenite to form a composite structure is lost, and the desired ferrite-martensite is lost. Cannot be obtained, and a sufficient fatigue limit ratio has not been obtained.

Since the content of Mn in the steel T-1 is less than the range of the present invention, the desired second phase, martensite, cannot be sufficiently obtained, and a low yield ratio has not been obtained. W-1
Since the content of C is smaller than the range of the present invention, the volume ratio of martensite in the structure is not sufficient, and a low yield ratio and a sufficient fatigue limit ratio are not obtained.

[0045]

As described in detail above, the present invention provides a composite structure high-strength cold-rolled steel sheet having excellent fatigue properties and a method for producing the same. By doing so, a significant improvement in fatigue properties can be expected while ensuring sufficient workability including elongation.
It can be said that the invention has high industrial value.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of preliminary experiments leading to the present invention in the relationship between the size of particles composed of Cu alone and the fatigue limit ratio.

FIG. 2 is a diagram showing the results of preliminary experiments leading to the present invention in the relationship between the concentration of B element and the fatigue limit ratio.

FIG. 3 is a diagram illustrating the shape of a fatigue test piece.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshiyasu Ukibo 2-6-3 Otemachi, Chiyoda-ku, Tokyo Inside Nippon Steel Corporation

Claims (5)

[Claims] 1. mass%, C: 0.03 to 0.20%, Si: 0.1 to 2.0%, Mn: 0.5 to 3.0%, P ≦ 0.02%, S ≦ 0.01%, Al: 0.005 to 0.1%, Cu: 0.2 to 2.0%, B: 0.0002 to 0.0020%, with the balance being Fe and unavoidable impurities Steel, the microstructure of which is a composite structure having ferrite as a main phase and martensite as a second phase, and the presence state of Cu in the ferrite phase is such that the size of particles composed solely of Cu is A high-strength cold-rolled steel sheet with a composite structure excellent in fatigue characteristics, which is in a solid solution state and / or a precipitate state of 2 nm or less. 2. The high-strength composite structure high-strength cold-rolled steel according to claim 1, wherein the steel further contains Ni: 0.1 to 1.0% by mass%. steel sheet. 3. The steel according to claim 1, further comprising one or two of Ca: 0.005 to 0.02% and REM: 0.005 to 0.2% by mass%. Claim 1
Or a composite structure high-strength cold-rolled steel sheet having excellent fatigue properties according to 2. 4. The steel further comprises, by mass%, Mo: 0.05 to 0.2%, V: 0.02 to 0.2%, Ti: 0.01 to 0.2%, Nb : 0.01 to 0.1%; Cr: 0.01 to 0.3%; Zr: 0.02 to 0.2%. 3. A high-strength composite structure high-strength cold-rolled steel sheet having excellent fatigue properties according to any one of 3. 5. A hot rolling of a steel slab having the component according to any one of claims 1 to 4 is performed in accordance with a conventional method except that a finish rolling is performed at an Ar ₃ transformation point or higher, and then in accordance with a conventional method. , Pickling, and cold rolling, and then performing continuous annealing, in the two-phase region of not less than the Ac ₁ transformation point and not more than the Ac ₃ transformation point.
After holding for 0 to 150 seconds, the microstructure is characterized by cooling to a temperature range of 400 ° C. or lower at a cooling rate of 20 ° C./s or higher. The microstructure has ferrite as a main phase and martensite as a second phase. A method for producing a composite structure high-strength cold-rolled steel sheet having excellent fatigue characteristics, wherein the size of particles composed of Cu alone in the ferrite phase is 2 nm or less.