JP3525812B2 - High strength steel plate excellent in impact energy absorption and manufacturing method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet which is suitable for a structural member manufactured by plastic working such as press forming and has excellent impact energy absorption, and a manufacturing method thereof.

[0002]

2. Description of the Related Art The social demand for ensuring collision safety of passenger cars has been increasing in recent years. Therefore, in order to secure a passenger space during a collision, various reinforcing members are attached to the vehicle body to increase the strength of the vehicle body. for that reason,
The vehicle weight tends to increase and the fuel economy tends to decrease. This conflicts with recent measures against global warming. Therefore, there is a movement to reduce the weight of the vehicle body by using a high-tensile steel plate. In particular, application of high-strength steel sheets having a pressure exceeding 500 MPa is being studied. Since ductility generally deteriorates as the strength of steel sheets increases, high-strength steel sheets with high ductility are desired.

To meet such needs, a low carbon steel sheet in which Si and Mn are added together is ferrite + austenite 2
After the phase region annealing, it is rapidly cooled to 350 to 550 ° C., and at that temperature stepwise cooling or holding for a short time to transform austenite into a part of bainite and finally to form a steel sheet consisting of ferrite + bainite + retained austenite (residual γ). Is
It is known that retained austenite undergoes strain-induced transformation during deformation and exhibits a large elongation.

For example, in JP-A-60-43430, C of 0.30 to 0.65% and S of 0.7 to 2.0% are used.
After heating a steel sheet containing i and 0.5 to 2.0% of Mn to the ferrite + austenite two-phase region in the annealing process, the temperature of 650 ° C. to 450 ° C. in the cooling process was 10 ° C.
Hold for 50 seconds or more once, and in the final product,
Disclosed is a method for manufacturing a steel sheet in which ferrite having a volume ratio of 10% or more, retained austenite, and the residual structure of one or two of martensite and bainite are disclosed.

In Japanese Patent Laid-Open No. 61-157625, 0.12 to 0.55% C and 0.4 to 1.8% S are used.
i and 0.2 to 2.5% of Mn as essential elements, and 0.5% or less of Cu, Cr, Ti and Nb, respectively.
V, Mo, P less than 0.1%, Ni less than 3%, 1
As a raw material, a steel sheet containing two or more kinds of steel is used, and after heating to a ferrite + austenite two-phase region in the same manner as in JP-A-60-43430, 500 ° C. during cooling.
A manufacturing method is disclosed in which the temperature is kept between ˜350 ° C. for 30 seconds to 30 minutes.

Further, in order to solve the shortage of stretch flange workability such as hole expansion, which is a drawback of steel sheets having a mixed structure as described above, a method for producing a retained austenitic steel sheet in which a part of Si is replaced by Al is proposed. It is disclosed in JP-A-5-70886.

[0007]

[Problems to be Solved by the Invention] Even with the development of steel sheets with high strength and excellent ductility as described above, there is an ever-increasing need for improved collision safety and weight reduction. Is required. However, even in the case of a steel sheet having high strength and excellent ductility as described above, there is an upper limit to the strength that can be actually used because the defective shape due to springback during forming increases as the strength increases. Further, the weldability may be deteriorated due to the increase in the amount of C accompanying the increase in strength.

Therefore, the present invention is intended to provide a steel sheet which has a high strength and at the same time has a high impact absorption energy at the time of collision, which is the object of the present invention.

[0009]

The gist of the present invention resides in the following (1), (2) and (3) high strength cold rolled steel sheet excellent in impact energy absorption and a method for producing the same (4 ). is there.

(1) C: 0.05 to 0.22 in mass%
5 %, Si: 2.0% or less, Al: 0.01 to 0.06
0 %, Mn: 0.5 to 4.0%, Ni: 0 to 5.0%,
P: 0.1% or less, S: 0.1% or less, N: 0.01%
Hereinafter, the balance consists of Fe and unavoidable impurities, and 1.5-3.0 × C ≦ Si + Al ≦ 3.5-5.0 ×
It has a chemical composition satisfying C and Mn + (Ni / 3) ≧ 1.0 (%), and the bake hardening amount of the steel sheet is 50M.
A cold-rolled steel sheet having Pa or more (here, the symbol of the elements in the formula represents the content of each element in the steel expressed in mass%).

(2) C: 0.05 to 0.22 in mass%
5 %, Si: 2.0% or less, Al: 0.01 to 0.06
0 %, Mn: 0.5 to 4.0%, Ni: 0 to 5.0%,
P: 0.1% or less, S: 0.1% or less, N: 0.01%
Hereinafter, Ti: 0.005 to 0.10% (however,
(48/14) N + (48/32) S + except 0.01 or less), Nb: 0.005 to 0.10%, V:
0.005 to 0.10% of one or more, 0.00 in total
5 to 0.10%, the balance consisting of Fe and unavoidable impurities, and 1.5-3.0 × C ≦ Si + Al ≦
3.5-5.0 × C, and Mn + (Ni / 3) ≧ 1.
A cold-rolled steel sheet having a chemical composition satisfying 0 (%) and having a bake hardening amount of 50 MPa or more.

(3) The steel sheet structure contains 5% or more of retained austenite and 5 to 15% of bainite.
Alternatively, the cold rolled steel sheet according to (2).

[0013]

(4) A steel slab having the chemical composition described in (1) or (2) above is heated and held at an Ac ₃ point or more, hot rolled, and wound in a temperature range of 300 to 720 ° C. , After pickling and cold rolling, in a continuous annealing step, heating to a temperature range of Ac ₁ point or more and Ac ₃ point or less to recrystallize, and then to 700 ° C. at a cooling rate of 10 ° C./s or less.
A method of performing slow cooling, holding in the temperature range of 550 to 350 ° C. for 30 seconds or more during the subsequent cooling, or performing slow cooling at a cooling rate of 100 ° C./min or less.

Here, the "bake-hardening amount" is a value measured by the method shown below. "Amount of retained austenite",
The “amount of bainite” is a value measured by the method described in “Embodiment of the Invention”.

The present inventors produced steel sheets having retained austenite having various compositions and structure balances on a laboratory scale, and simulated deformation of structural members (hereinafter referred to as members) in the collision of automobiles. A test was conducted to investigate the impact absorbed energy. The details of the experiment will be described below.

Steel pieces having a composition of steel A and steel B shown in Table 1 and having a thickness of 25 mm were hot-rolled, pickled, cold-rolled, annealed and temper-rolled in a laboratory, and then subjected to a tensile test and A shaft crush test of the member was performed. Hot rolling the billet at 1200 ℃
After heating for 30 minutes, it was performed in the temperature range of 900 ° C. to 800 ° C. to have a plate thickness of 5 mm. After hot rolling, cool to 600 ° C with water spray cooling, hold for 30 minutes, 20 ° C
The coil winding in the actual manufacturing was simulated by cooling the furnace at 1 / hour. The hot-rolled sheet was pickled to remove the scale and then cold-rolled to a sheet thickness of 1.2 mm. This cold-rolled sheet was soaked at 820 ° C for 30 seconds and then cooled to 700 ° C at 3 ° C.
The sample was gradually cooled at a heating rate of 1 / sec, then cooled to the overaging temperature at 50 ° C./sec, held at that temperature, and further cooled to room temperature at 10 ° C./sec. The overaging temperature was 250 to 600 ° C., the holding time was 15 to 300 seconds, and various combinations of conditions were tested. After annealing, temper rolling was performed at an elongation of 1%.
A tensile test was performed after temper rolling.

Further, in order to measure the bake-hardening amount (BH amount), a tensile prestrain of 2% was applied according to JIS G 3135, and after heat treatment equivalent to coating baking for 20 minutes at 170 ° C. was performed. A tensile test was conducted. "Bake-hardening amount" is
The value obtained by subtracting the prestrain load from the strain aging yield load was divided by the cross-sectional area of the parallel part of the test piece before prestrain.

The impact absorption energy was measured as follows. In the test piece for the axial crush test shown in FIG. 1, the steel plate manufactured as described above was formed into a hat shape by press forming, and steel plates of the same material were spot welded at a pitch of 20 mm. The plastic strain of the hat wall was about 2%. This sample was heat-treated at 170 ° C. for 20 minutes, which corresponds to baking, and then subjected to a shaft crushing test. In the axial crush test, a weight of 350 kg was dropped from a height of 3 m, and 100 m from the load-displacement curve.
The energy required to crush m was calculated. The composition A having a tensile strength of 580 to 620 MPa and the composition B having a tensile strength of 680 to 720 MPa were extracted, and the relationship between the bake hardening amount and the impact absorption energy was examined. As shown in FIG. 2, the larger the bake-hardening amount is, the larger the shock absorption energy is, which is preferable. In particular, when the bake-hardening amount is 50 MPa or more, the impact absorption energy increases remarkably. For example, a steel sheet having a bake hardening amount of 50 MPa of the composition A exhibits the same impact absorption energy as a steel sheet having a bake hardening amount of the composition B of 20 MPa. Since the latter has a tensile strength higher than that of the former by 100 MPa, bake hardening is considered to have more than a simple strength increasing effect.

Therefore, the basic idea for achieving both the formability governed by tensile strength such as springback and the axial crush absorption energy is to secure the tensile strength of the steel sheet and then to obtain a high bake hardening amount. Is to give.

Next, the relationship between the metal structure and the tensile test characteristics of the group of composition A is shown in FIG. 3, and the following important findings were obtained.

The larger the amount of bainite (area ratio), the higher the amount of bake hardening.

On the other hand, tensile strength (TS) and elongation value (E
The product (strength-ductility balance) with L) is excellent when the bainite amount is in the range of 5 to 15%, and decreases when the bainite amount is less than 5% or exceeds 15%.

Therefore, the amount of bainite is 5 to 15%.
When adjusted to 1, the bake hardening amount is large, that is, the impact absorption energy is large, and the strength-ductility balance is good.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The steel sheet of the present invention and the method for producing the same will be described below in detail.

C: The strongest austenite stabilizing element, which is one of the essential elements of the present invention. In order to stabilize austenite at room temperature, 1% or more of C must be contained in austenite. When the total C content is 0.05% or more, C can be concentrated in the retained austenite by optimizing the heat cycle of hot rolling or annealing. A higher strength steel plate can be manufactured by adding a larger amount of C. However, if it exceeds 0.3%, the strength becomes too high, so that it becomes difficult to form a component due to problems such as a large springback amount. Since the weldability also deteriorates, the upper limit was made 0.3% and the range of C was made 0.05 to 0.3%.
It is preferably 0.08 to 0.23%.

Si: Si, like Al, is a ferrite stabilizing element, and in the cooling process after hot rolling or in the two-phase annealing of a cold rolled steel sheet, the volume ratio of ferrite is increased to equilibrate the austenite phase. Functions to increase the C concentration of. Further, it has a function of suppressing the precipitation of carbide during bainite transformation and concentrating C in austenite. At the same time, Si has a function of strengthening ferrite. However, if the Si content exceeds 2.0%, the surface quality deteriorates due to the high Si scale peculiar to the Si-added steel sheet, so the upper limit was made 2.0%. Further, the content is optimized in relation to Al.

Al: Al, like Si, is a ferrite stabilizing element and has the effect of increasing the C concentration in the austenite phase, and limits the amount added in relation to Si. When producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, it is preferable to positively add Al that does not impair the plating wettability. Further, it is used as a deoxidizing agent during steel making, and 0.01% or more is necessary to obtain a sufficient deoxidizing effect. When it exceeds 2.0%, inclusions increase in the steel sheet and the ductility is impaired, so this was made the upper limit.

Balance of Si, Al and C: In order to secure sufficient retained austenite, Si and Al are in total,
It is necessary to contain "1.5-3.0 x C" or more. However, the excessive addition of Si and Al excessively stabilizes the retained austenite, and deteriorates the ductility rather than the ductility especially when the C content is high. Therefore, the upper limit of "Si + Al" is "3.5-
5.0 × C ”. Moreover, since Al has a large effect of stabilizing the retained austenite, 1.25 × Si
(%) Or less is preferable.

Mn: An austenite-stabilizing element, which is one of the essential elements of the present invention, and its addition amount is regulated in relation to Ni. In order to stabilize austenite and keep it at room temperature, “Mn + (Ni / 3)” is 1.0
It is necessary to add it so that the content is at least%. Mn is also added for the purpose of fixing S in steel as MnS and preventing hot embrittlement, so the lower limit was made 0.5%. Also, 4.0
%, The steel sheet becomes too hard and high ductility cannot be obtained, so the upper limit was made 4.0%.

Ni: Like Mn, it may be added as an austenite stabilizing element. However, the effect of stabilizing austenite is smaller than that of Mn, which is about 30% of Mn. Further, it is more expensive than Mn, and basically Mn may be added to stabilize austenite.
However, when a plated steel sheet is manufactured by a continuous hot-dip galvanizing line, there is an effect of preventing Mn oxide from being generated on the surface of the steel sheet and deteriorating the wettability of the plating. Regulated by the total with Mn content, "Mn + (Ni / 3)" is 1.0%
It is necessary to be above. If the Ni content exceeds 5.0%, the product cost becomes too high, so this was made the upper limit.

P: The lower P is, the better. However, P has the effect of strengthening the solid solution of ferrite,
It may be added to obtain the effect. If the P content exceeds 0.1%, the weldability deteriorates, so 0.1%
Is the upper limit.

S: S is preferably low. When the S content is high, the amount of MnS precipitated increases, which not only hinders the ductility but also consumes Mn added as an austenite stabilizing element as a precipitate, so the content is limited to 0.1% or less.

N: N is preferably as low as possible in the current steelmaking technology. The upper limit was 0.01% to ensure workability.

Ti, Nb, V: All are elements added as necessary. These elements are carbide forming elements and form fine precipitates to strengthen the steel sheet. When any element is added, the range of 0.005 to 0.10% is preferable. When adding two or more compounds in combination, the total amount is. 005 to 0.10% is preferable.

Further, Ti is particularly easy to bond with N in steel,
TiN is deposited in preference to the deposition of AlN. Prevents slab cracking due to AlN precipitation and
In order to obtain the effect of refining the hot rolled sheet by
It is preferable to add it so that / N ≧ 3.

Bake-hardening amount: As described above, the higher the bake-hardening amount, the higher the impact absorption energy. The effect is remarkably large at 50 MPa or more. The reason for this is not clear, but it is presumed as follows. In the present invention, "bake hardening amount" means "means for solving the problem".
It is the amount obtained by the tensile test specified in the above.

Bake hardening is a phenomenon that occurs because dislocations are fixed by carbon atoms. The stronger the dislocations are fixed, the lower the movable dislocation density and the greater the strain rate dependence of deformation resistance. The higher the bake-hardening amount, the higher the deformation resistance in a quasi-static tensile test such as the tensile test by JIS No. 5 test piece, and also the higher deformation resistance in the deformation at the time of collision with a high strain rate, It is presumed that the impact absorption energy will be significantly increased.

In order to exert such an effect, a bake-hardening amount of at least 50 MPa or more is necessary.
0 MPa or more is desirable.

Volume Ratio of Retained Austenite: Since the ductility of the steel of the present invention improves with an increase in the volume ratio of the retained austenite contained in the product, the volume ratio of the retained austenite is preferably 5% or more. Thereby, further improvement of ductility due to strain-induced transformation of austenite can be expected. Further, the upper limit of the retained austenite is preferably 25% or less, particularly when high moldability is obtained. The balance is mainly ferrite or ferrite + bainite.

The volume ratio of retained austenite is determined by exposing the center plane of the plate thickness parallel to the steel plate surface by chemical polishing.
It is calculated by the following formula by X-ray diffraction using CoKα ray.

Volume ratio of retained austenite (%) = {1 / (1
+ K ⁻¹ × I α × I γ ⁻¹ )} × 100 In the above formula, K is a constant that can be obtained by a standard sample made of a mixed powder of SUS304 and pure iron, and Iα is (211) of ferrite.
The X-ray integrated intensity of the plane, Iγ, is (22) of the retained austenite.
It represents the X-ray integrated intensity of the 0) plane.

Area ratio of bainite: As described above,
The bake hardening amount increases with an increase in the area ratio of bainite, but the strength-ductility balance decreases. The area ratio of bainite is 5 to 1 in order to achieve both of these contradictory characteristics.
5% is desirable. The area ratio of bainite was determined by observing a steel plate cross section perpendicular to the rolling direction with a scanning electron microscope and measuring the area occupied by the bainite structure.

[0044]

[0045]

[0046]

[0047]

Hot-rolling conditions: In the case of cold-rolled steel sheets, the metal structure is mainly determined in the continuous annealing process, so there are few restrictions on the hot-rolling conditions, but the winding temperature is restricted for the following reasons. In the case of the steel of the present invention, when it is wound at a low temperature, it is hardened and hardened, so that subsequent pickling and cold rolling become difficult. Also,
When wound at high temperature and cementite coarsening, pickling becomes soft, while the cold rolling is facilitated, too much time to cementite during soaking annealing forms a solid solution again, sufficient austenite can not remain . Therefore, 300 to 7
Limited to 20 ° C. It is preferable to wind at a low temperature as long as it does not interfere with pickling and cold rolling. Preferably 550-65
0 ° C is good.

Continuous Annealing Conditions: First, in order to make C into austenite by forming two phases of "ferrite + austenite", it is heated in a temperature range from Ac ₁ point to Ac ₃ transformation point. If the heating temperature is too low, it takes too long to re-dissolve cementite, and if it is too high, the volume ratio of austenite becomes too large and the C concentration in austenite decreases . Therefore, it is desirable to carry out soaking in the temperature range of 800 to 850 ° C. In order to further increase the C concentration in austenite by further soaking and then gradually cooling to grow ferrite,
Cooling rate to 700 ° C. is not more than 10 ° C. / s. Further, in the temperature range before entering the overaging treatment zone, the cooling rate is preferably 50 ° C./s or more to suppress the pearlite transformation of austenite.

In the overaging zone, the temperature is maintained at 550 to 350 ° C. for 30 seconds or more or is gradually cooled at a cooling rate of 100 ° C./min or less to transform C into austenite while condensing C into austenite. Good to promote. In the temperature range of 550 to 350 ° C., the retained austenite is slightly reduced when held for more than 250 seconds.
Seconds or less are preferable. If it exceeds 550 ° C, bainite transformation does not occur, and if it is less than 350 ° C, it becomes lower bainite, and C concentration in austenite hardly occurs. In terms of material characteristics, the cooling rate after the overaging zone need not be particularly limited, but forced cooling may be performed from the viewpoint of shortening the annealing line length.

In order to produce a hot dip plated steel sheet,
You may perform the said heat processing using a continuous hot dip coating line. An alloying heat treatment may be performed in order to achieve galvannealing.

[0052]

Examples (Example 1) Steels A to M whose chemical compositions are shown in Table 1
The 25 mm thick steel slab was hot-rolled, pickled, cold-rolled, annealed and temper-rolled in the laboratory, and then subjected to a tensile test and an axial crush test of the member shown in FIG. Hot rolling
After heating the billet to 1200 ° C for 30 minutes, the temperature of 900 ° C to 8
The rolling was performed in the temperature range of 00 ° C. and rolled to a plate thickness of 5 mm. After hot rolling, the material was cooled to 600 ° C. by water spray cooling, held for 30 minutes, and furnace cooled at 20 ° C./hour to simulate coil winding in actual machine manufacturing. The hot-rolled sheet was pickled to remove the scale and then cold-rolled to a sheet thickness of 1.2 mm. The cold-rolled sheet was soaked at 820 ° C. for 30 seconds, then gradually cooled to 700 ° C. at 3 ° C./second, and then 4
The mixture was cooled to 00 ° C at 30 ° C / sec, kept at that temperature for 150 seconds, and further cooled to room temperature at 10 ° C / sec. After annealing, temper rolling with an elongation of 1% was performed. Table 2 shows the mechanical properties and metallographic structure of these steel sheets. The bake hardening amount, the amount of retained austenite (residual γ), and the amount of bainite were determined by the above-mentioned method.

[0053]

[Table 1]

[0054]

[Table 2]

In sample No. 3, since C is 0.040%, which is lower than the range of the present invention, residual austenite is scarcely present and the elongation is poor. Test No. 5 has a C content of 0.33%, which is higher than the range of the present invention, and therefore has a tensile strength of 1200 MPa or more, which is too high strength and not suitable for press molding applications. In the trial number 6, the amount of “Si + Al” is lower than the range of the present invention,
Since C is not sufficiently concentrated in the austenite phase during the two-phase annealing, the amount of retained austenite is small and the elongation is poor. On the other hand, in the sample No. 7, the amount of “Si + Al” is too higher than the range specified in the present invention, so that the C concentration in the retained austenite becomes too high and is excessively stabilized, so that the amount of retained austenite is large. However, the growth is low. Sample No. 9 has Mn of 0.31%, which is lower than the range of the present invention, and the austenite is not stabilized. Therefore, when cooled to room temperature, martensitic transformation occurs and residual austenite disappears, so that elongation is poor.

Trial Nos. 1, 2, 4, 8, 10, 1 of the present invention example
1, 12 and 13 have excellent strength-ductility balance (TS
X EL).

(Example 2) A steel piece having a thickness of 25 mm of steel A having a chemical composition shown in Table 1 was hot-rolled, pickled, cold-rolled, annealed and tempered in a laboratory and then stretched. The test and the axial crush test of the member were performed. Hot rolling is performed by soaking the billet at 1200 ° C for 30 minutes, and then 900 ° C to 800 ° C.
It was carried out in the temperature range of ° C and the plate thickness was set to 5 mm. After hot rolling, cool to 600 ° C with water spray cooling,
The coil winding in the actual production was simulated by holding the temperature for 20 minutes and cooling the furnace at 20 ° C./hour. The hot-rolled sheet has a thickness of 1.2 m by cold rolling after removing the scale by pickling.
m. This cold-rolled sheet was soaked at the soaking temperature shown in Table 3 for 30 seconds, then gradually cooled down to 700 ° C at 3 ° C / second, then cooled to the overaging start temperature at 30 ° C / second, and then overheated. It cooled to the aging completion temperature at the average cooling rate in the same table, and further cooled to room temperature at 10 ° C./sec. After annealing, elongation is 1%
Was temper-rolled.

The trial numbers 1, 14 to 19 in Table 3 are examples in which the overaging treatment is performed at a constant temperature.

Table 4 shows the mechanical properties, the metal structure and the axial crush absorption energy when the continuous annealing conditions were changed as shown in Table 3.

[0060]

[Table 3]

[0061]

[Table 4]

In the sample No. 14, the soaking temperature of continuous annealing is higher than the range specified in the present invention, so that it is cooled from the austenite single phase region, so that C is not concentrated and no residual austenite remains. Since the bake-hardening amount is high, the axial crush absorption energy is high, but the elongation is extremely small and it cannot withstand press forming. Sample Nos. 15 and 22 have a short overaging time, so that residual austenite does not remain and the bake hardening amount is small.
Both elongation and energy absorption are poor. Since the overaging time of Sample No. 17 is 300 seconds, which is slightly long, bainite transformation tends to proceed and the residual amount tends to slightly decrease. Trial No. 18, 19, 23
Since the overaging temperature is outside the range of the present invention, residual austenite is not obtained. Therefore, the growth of trial numbers 18 to 19 is inferior.

The trial numbers 1, 16, 20, and 21 of the examples of the present invention have extremely good elongation and absorbed energy.

[0064]

[0065]

[0066]

[0067]

[0068]

According to the present invention, the strength of the steel sheet is not unnecessarily high. In other words, the impact hardening can be achieved by defining the bake hardening amount of the steel sheet without causing defects such as springback during forming. A steel sheet excellent in absorbed energy can be obtained. By applying this steel sheet to structural members such as automobiles, it is possible to improve collision safety and reduce weight.

[Brief description of drawings]

FIG. 1 is a schematic view of a hat-shaped member used for evaluating impact absorption energy.

FIG. 2 is a diagram showing a relationship between a bake-hardening amount and impact absorption energy.

[FIG. 3] Area ratio of bainite and bake hardening amount and strength-
It is a figure which shows the relationship with a ductility balance.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-10-158735 (JP, A) JP-A-7-252592 (JP, A) JP-A-10-130776 (JP, A) JP-A-5- 179345 (JP, A) JP-A-5-70886 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 C21D 9/46

Claims (4)

(57) [Claims] 1. In mass%, C: 0.05 to 0.225 %,
Si: 2.0% or less, Al: 0.01 to 0.060 %,
Mn: 0.5 to 4.0%, Ni: 0 to 5.0%, P:
0.1% or less, S: 0.1% or less, N: 0.01% or less, the balance consisting of Fe and unavoidable impurities, and 1.5-3.0 × C ≦ Si + Al ≦ 3.5-5 0.0 ×
It has a chemical composition satisfying C and Mn + (Ni / 3) ≧ 1.0 (%), and the bake hardening amount of the steel sheet is 50M.
A high-strength cold-rolled steel sheet excellent in impact energy absorption, characterized by having Pa or more. 2. In mass%, C: 0.05 to 0.225 %,
Si: 2.0% or less, Al: 0.01 to 0.060 %,
Mn: 0.5 to 4.0%, Ni: 0 to 5.0%, P:
0.1% or less, S: 0.1% or less, N: 0.01% or less, and Ti: 0.005 to 0.10% (however, (4
8/14) N + (48/32) S + except 0.01 or less), Nb: 0.005 to 0.10%, V: 0.0
05-0.10% of one or more, 0.005-total
0.10% content, the balance consisting of Fe and unavoidable impurities, and 1.5-3.0 × C ≦ Si + Al ≦ 3.5
−5.0 × C, and Mn + (Ni / 3) ≧ 1.0
(%), A high-strength cold-rolled steel sheet excellent in impact energy absorption, characterized in that the bake hardening amount of the steel sheet is 50 MPa or more. 3. A high-strength cold-rolled steel sheet excellent in impact energy absorption according to claim 1, wherein the steel sheet structure contains 5% or more of retained austenite and 5 to 15% of bainite. 4. A steel slab having the chemical composition defined in claim 1 or 2 is heated and held at an Ac ₃ point or higher, hot-rolled, wound in a temperature range of 300 to 720 ° C., pickled, after the cold rolling, the continuous annealing process, Ac ₁ point or more, then heated to a temperature range below Ac ₃ point recrystallized, following
Slowly cool to 700 ° C at a cooling rate of 10 ° C / s or less, and then hold for 30 seconds or more in the temperature range of 550 to 350 ° C during the subsequent cooling, or gradually cool at a cooling rate of 100 ° C / min or less. And a method for producing a high-strength cold-rolled steel sheet excellent in impact energy absorption.