JP3530356B2 - Good workability high-strength cold-rolled steel sheet with high dynamic deformation resistance for impact absorption at the time of collision and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for automobile members and the like, and at the time of collision showing high dynamic deformation resistance which can contribute to ensuring safety of passengers by efficiently absorbing impact energy at the time of collision. The present invention relates to a high-strength cold-rolled steel sheet for impact absorption and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, occupant protection in the event of an automobile collision has been recognized as the most important performance of an automobile, and there is an increasing expectation for a material having a high high-speed deformation resistance in order to cope with it. For example, in the case of a frontal collision of a passenger car, if such a material is applied to a member called a front side member, the energy of the shock is absorbed by the member being crushed, and the shock to the occupant can be softened.

Since the strain rate of deformation which each part receives at the time of a collision of an automobile reaches up to about 10 ³ (1 / s), when considering the shock absorbing performance of a material, dynamic strain in such a high strain rate range is considered. It is necessary to clarify the deformation characteristics. At the same time,
At the same time, it is indispensable to achieve the weight reduction of the automobile body aiming at energy saving and reduction of CO ₂ emission, and for this reason, there is a growing need for effective high strength steel sheets.

[0004] For example, the present inventors have found that CAMP-ISIJ
Vol. 9 (1996) P.I. 1112 to 1115,
Reported on high speed deformation properties and impact energy absorption capacity of high-strength thin steel sheet, in which the dynamic strength at high strain rate region of the order of 10 ³ (1 / s) is 10 ^-3 (1 / s) A large increase in static strength at low strain rate, a change in strain rate dependence of deformation resistance due to the strengthening mechanism of the material, among which TRIP (transformation induced plasticity) type steel and DP ( It has been reported that a ferrite / martensite two-phase type steel has both excellent formability and impact absorbing ability as compared with other high strength steel sheets.

Further, Japanese Patent Application Laid-Open No. 7-18372 discloses a high-strength steel sheet containing retained austenite and having excellent impact resistance and a method for producing the same. Although it is disclosed that the problem can be solved only by raising, it is not clear how to control the properties of the retained austenite other than the amount of the retained austenite in order to improve the impact absorption capacity.

[0006]

As described above, although the dynamic deformation characteristics of the material constituting the members that affect the absorption of impact energy in the event of an automobile collision are being elucidated little by little, it is for automobile parts having excellent impact energy absorption ability. It has not yet been clarified what characteristics the steel material should be focused on and what criteria should be used for the material selection. Further, automobile parts are formed into a required part shape by press forming a steel material, and then are generally baked in a paint and then incorporated into an automobile to face an actual collision phenomenon. However, it has not been clarified yet what kind of steel material strengthening mechanism is suitable for improving the impact energy absorption capacity at the time of collision of the steel material after such pre-deformation + baking treatment.

[0007]

DISCLOSURE OF THE INVENTION The present invention is a steel material that is used by being formed into a part that absorbs impact energy at the time of collision, such as a front side member, and a high-strength steel sheet that exhibits a high impact energy absorption capacity. It is intended to provide a manufacturing method. The gist of the present invention is as follows.

(1) C: 0.04% or more by weight%, 0.
3% or less, 0.5 of Si or Al or both in total
% Or more and 3.0% or less, and one or more of Mn, Ni, Cr, Cu, Mo in a total amount of 0.5% or more and 3.5% or less, and the balance consisting of Fe and inevitable impurities. The cold-rolled steel sheet thus obtained has a microstructure containing ferrite and bainite, which is a composite structure with a third phase containing either of them as a main phase and containing 3% or more of retained austenite by volume fraction. Amount of solid solution [C] and average Mn equivalent of steel material {Mneq = Mn + (Ni + Cr + Cu
+ Mo) / 2} (M = 678-428)
X [C] -33xMneq) is -140 or more and less than 70 , has a tensile strength of 588 MPa or more , and after pre-deformation of more than 0% and 10% or less by equivalent strain to the steel material, it is 5x1.
Mean value σdyn (MPa) of deformation stress and 5 × 10 ^{−4 to} 5 × 10 ⁻ in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 0 ^{2 to} 5 × 10 ³ (1 / s). ³ (1 /
s) the strain is 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s) when the strain is deformed in the strain rate range and the difference in the average value σst (MPa) of the deformation stress in the equivalent strain range of 3 to 10% is 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s). Maximum stress TS (M in a static tensile test measured in the velocity range
Pa) expressed by (σdyn−σst) ≧ −
A good workability, high strength cold rolled steel sheet having high dynamic deformation resistance for impact shock absorption, which satisfies 0.272 × TS + 300.

(2) Absorption at impact with high dynamic deformation resistance as described in (1), further containing one or more of Nb, Ti and V in a total amount of 0.3% by weight or less.
Use good workability high-strength cold-rolled steel sheet. (3) An impeller having a high dynamic deformation resistance as described in (1) or (2), which further contains 0.2% by weight or less of P.
Good workability and high strength cold rolled steel sheet for shock absorption at impact .

(4) B: 0.01% by weight or less is further contained, and good workability for impact shock absorption with high dynamic deformation resistance according to any one of (1) to (3) is provided. High strength cold rolled steel sheet. (5) The residual austenite volume fraction of the steel material after pre-deformation of more than 0% and 10% or more is more than 2.5%, and
The high dynamics according to any one of (1) to (4), characterized in that the ratio of the retained austenite volume fraction before pre-deformation to the retained austenite volume fraction after pre-deformation is 0.4 or more. Good workability and high-strength cold-rolled steel sheet for impact absorption at the time of collision with deformation resistance.

(6) The ratio of the average grain size of retained austenite in the microstructure of the finally obtained cold rolled steel sheet to the average grain size of ferrite or bainite as the main phase is 0.6.
Good workability high-strength cold-rolled steel sheet for impact absorption during collision having high dynamic deformation resistance according to any one of (1) to (5), characterized in that : (7) Weight%, C: 0.04% or more and 0.3% or less, S
One or both of i and Al total 0.5% or more 3.0
% Or less, one or two of Mn, Ni, Cr, Cu, Mo
A casting slab that contains 0.5% or more and 3.5% or less in total of the seeds or more, and the balance of Fe and unavoidable impurities is directly sent to the hot rolling step as cast, or after being cooled once and then heated again, When pickling the cold rolled hot rolled steel sheet which was hot rolled and wound and then annealed in the continuous annealing step to make the final product,
0.1 × (Ac ₃ −Ac ₁ ) + Ac ₁ ° C. or higher Ac ₃ +5
After annealing for 10 seconds to 3 minutes at a temperature of 0 ° C. or less, 1 to 10
Cooling to a primary cooling stop temperature in the range of 550 to 670 ° C at a primary cooling rate of ° C / sec, followed by 10 to 200 ° C /
After cooling to a secondary cooling stop temperature of 320 ° C. to 500 ° C. at a secondary cooling rate of sec, hold at a temperature range of 320 ° C. to 500 ° C. for 15 seconds to 20 minutes, cool to room temperature, and finally obtain The cold-rolled steel sheet has a microstructure containing ferrite and bainite, which is a composite structure with a third phase containing either of them as the main phase and a retained austenite content of 3% or more, and a tensile strength of 588 MPa or more. It has an average of solute (C) the amount and the steel in the residual austenite Mn eq {Mneq = Mn + (Ni + Cr + Cu + Mo) /
2} value (M = 678-428 × [C] −
33 × Mneq) is 70 or more and 180 or less, and 5 × after giving a pre-deformation of more than 0% and 10% or less with equivalent strain to the steel material.
Mean value σdyn (MPa) of deformation stress and 5 × 10 ^{−4 to} 5 × 10 ⁻ in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 10 ^{2 to} 5 × 10 ³ (1 / s). ³ (1
/ S) when deformed in the strain rate range of 3 to 10% in the equivalent strain range of the deformation stress average value σst (MPa) is 5 × 10 ^{-4 to} 5 × 10 ^-3 (1 / s) Maximum stress TS in a static tensile test measured in the strain rate range
Expression (σdyn−σst) expressed by (MPa)
A method for producing a good workability high strength cold rolled steel sheet for impact shock absorption having high dynamic deformation resistance, characterized by satisfying ≧ −0.272 × TS + 300.

(8) The impact absorption at the time of collision with high dynamic deformation resistance as described in (7), which further contains one or more of Nb, Ti and V in a total amount of 0.3% by weight or less.
Manufacturing method of use good workability high-strength cold-rolled steel sheet. (9) The impact having a high dynamic deformation resistance according to (7) or (8), further containing 0.2% by weight or less of P.
A method for manufacturing a high-workability, high-strength cold-rolled steel sheet for impact shock absorption .

(10) Good workability for impact shock absorption at high impact having high dynamic deformation resistance as set forth in any one of (7) to (9), further containing 0.01% by weight or less of B. Manufacturing method of high strength cold rolled steel sheet.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A shock absorbing member such as a front side member of an automobile at the time of collision is manufactured by bending or press forming a steel plate. The impact at the time of collision of the automobile is applied after being processed in this way and generally after being baked. Therefore, a steel sheet that exhibits a high impact energy absorption capacity after being processed and treated as described above is required.

As a result of the research conducted by the present inventors, it has been found that it is suitable for a high-strength steel sheet having excellent impact absorption characteristics that the steel sheet in such a formed actual member contains an appropriate amount of retained austenite. . That is, the optimum microstructure contains ferrite and bainite that are easily solid-solution strengthened by various substitutional elements, and the residual austenite that transforms into hard martensite during deformation is contained as the main phase in either volume. If the rate includes 3% or more,
It was found to exhibit high dynamic deformation resistance. Further, even when martensite particles are included in the third phase of the initial microstructure, if other conditions are satisfied, the good workability high-strength cold-rolled steel sheet having high dynamic deformation resistance, which is the object of the present invention, is obtained. Can be obtained.

The reasons for limiting each component of the cold rolled steel sheet specified in the present invention are as follows. C: C is the most important element in the present invention because it is the cheapest element that contributes to the stabilization of austenite necessary for stabilizing and remaining austenite at room temperature. The average C content of steel not only affects the retained austenite volume fraction that can be secured at room temperature, but also improves the stability of retained austenite during processing by concentrating it in untransformed austenite during the manufacturing heat treatment. It can be done. However, if the added amount is less than 0.04% by weight, the finally obtained retained austenite volume fraction cannot be 3% or more, so 0.04% by weight was set as the lower limit. On the other hand, the retained austenite volume fraction that can be ensured increases as the average C content of the steel material increases, and it becomes possible to secure the stability of the retained austenite while securing the retained austenite volume ratio. However, if the amount of C added to the steel material becomes excessively large, the strength of the steel material is increased more than necessary, and not only the formability such as press working is hindered but also the dynamic stress increase is caused as compared with the static strength increase. The upper limit of the amount of C added is set to 0.3% by weight because the use of steel as a part is restricted due to the impediment and deterioration of weldability.

Al, Si: Al and Si are both stabilizing elements of ferrite, and have the function of improving the workability of the steel material by increasing the ferrite volume ratio. Further, since both Al and Si suppress the generation of cementite and enable effective concentration of C in austenite, it is unavoidable to leave a proper amount of austenite at room temperature. Is an additional element. Examples of additional elements having such a cementite formation suppressing function include P, Cu, Cr, and Mo in addition to Al and Si. Proper addition of such elements is expected to have the same effect. . However, when the total amount of one or both of Al and Si is less than 0.5% by weight, the effect of suppressing the formation of cementite is not sufficient, and most of the added C that is most effective for stabilizing austenite is a carbide. The amount of residual austenite volume ratio required for the present invention cannot be secured, or the manufacturing conditions necessary for securing retained austenite are not suitable for the conditions of mass production process. %. Also, Al and S
When the amount of one or both of i exceeds 3.0% by weight, not only the ferrite or bainite as the matrix phase is hardened or embrittled, but also the increase of the deformation resistance due to the increase of the strain rate is hindered. Since the workability of the steel material is lowered, the toughness is lowered, the cost of the steel material is increased, and the surface treatment characteristics such as chemical conversion treatment property are remarkably deteriorated, 3.0% by weight is set as the upper limit value.

Mn, Ni, Cr, Cu, Mo: Mn, N
i, Cr, Cu and Mo are all austenite stabilizing elements and are effective elements for stabilizing austenite at room temperature. In particular, when the amount of C added is limited from the viewpoint of weldability, it is possible to effectively retain austenite by adding an appropriate amount of such an austenite stabilizing element. Further, these elements have an effect of suppressing the formation of cementite, though not so much as Al or Si, and also have a function of helping the concentration of C in austenite. Further, these elements also have a function of enhancing the dynamic deformation resistance at high speed by solid-solution strengthening the matrix ferrite or bainite together with Al and Si. However, one or two of these elements
If the total addition of at least one species is less than 0.5% by weight, it becomes impossible to secure the necessary retained austenite and the strength of the steel material decreases, making it impossible to achieve effective weight reduction of the vehicle body. It was 0.5% by weight. On the other hand, when the total amount of these exceeds 3.5% by weight, the matrix ferrite or bainite is hardened, which not only hinders the increase of the deformation resistance due to the increase of the strain rate, but also the workability of the steel material. The upper limit was set to 3.5% by weight in order to lower the toughness and the steel cost.

Nb, Ti, V: In addition, Nb, Ti, V added as necessary can strengthen the steel material by forming carbides, nitrides or carbonitrides, but the total of them. When it exceeds 0.3% by weight, a large amount of carbides, nitrides or carbonitrides are precipitated in the matrix of ferrite or bainite grains or in grain boundaries, and become a source of mobile dislocations during high speed deformation. , It becomes impossible to obtain high dynamic deformation resistance. Further, the formation of carbides inhibits the concentration of C in the retained austenite, which is the most important for the present invention, and wastes C, so the upper limit is 0.3% by weight.
And

P: Further, P added as necessary is effective for increasing the strength of the steel material and securing retained austenite as described above. However, when P is added in excess of 0.2% by weight, Not only does this lead to an increase in the cost of the steel material, but it also increases the deformation resistance of the main phase ferrite and bainite more than necessary, and hinders the increase of the deformation resistance during high-speed deformation. Further, 0.2 wt% is set as the upper limit because deterioration of resistance to cracking due to placement, deterioration of fatigue characteristics and deterioration of toughness are caused.

B: B, which is added as necessary, is effective for strengthening grain boundaries and strengthening the steel material, but when the addition amount exceeds 0.01% by weight, the effect is saturated. In addition, the steel plate strength is increased more than necessary, the increase of the deformation resistance at the time of high-speed deformation is hindered, and the workability of parts is also decreased. Therefore, the upper limit was made 0.01% by weight. Next, as a result of experiments and studies by the present inventors, the amount of pre-deformation corresponding to the forming process of the impact absorbing member such as the front side member may reach up to 20% or more depending on the part in the member. However, most of the equivalent strains are more than 0% and less than 10%, and by grasping the effect of pre-deformation in this range, it is possible to estimate the behavior of the entire member after pre-processing. I found it. Therefore, in the present invention, as the amount of pre-deformation given to the member during processing, a strain of not less than 0% and not more than 10% in equivalent strain is selected.

Further, the shock absorbing member such as the front side member characteristically has a hat-shaped cross section,
As a result of the analysis by the present inventors of the deformation at the time of high-speed collision crushing of such a member, the deformation has advanced to a high strain of 40% or more at the maximum, but about 70% of the total absorbed energy.
It has been found that% or more is absorbed in the strain range of 10% or less of the high-speed stress-strain diagram. Therefore, the dynamic deformation resistance during high-speed deformation at 10% or less was adopted as an index of the impact energy absorption capacity at high speed. Especially, since the range of 3% to 10% is the most important as the strain amount,
The equivalent strain during high-speed tensile deformation was used as an index of impact energy absorption capacity with an average stress σdyn in the range of 3% to 10%.

The average stress σdyn (MPa) of 3% to 10% at the time of high-speed deformation is the static tensile strength (5 ×) of the steel material.
Maximum stress in a static tensile test measured in a strain rate range of 10 ^{−4 to} 5 × 10 ⁻³ (1 / s): TS (MP
It generally becomes larger as a)) rises.
Therefore, increasing the static tensile strength of the steel material directly contributes to the improvement of the impact energy absorption capacity of the member. However, if the strength of the steel material increases, the formability of the member deteriorates, and it becomes difficult to obtain the required member shape. Therefore,
A steel material having the same tensile strength (TS) and high σdyn is desirable. In particular, the strain level during processing of parts is mainly 10
% Or less, it is important for improving the moldability that the stress in the low strain region, which is an index of the moldability such as shape fixability during molding into a member, is low. Therefore σdyn and 5
The larger the difference in the average value σst (MPa) of the deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of × 10 ^{-4 to} 5 × 10 ^-3 (1 / s), the more static the static. Can be said to have excellent formability and to have a high impact energy absorption capacity dynamically. In this relationship, in particular (σdyn−σst) ≧ −
A steel material satisfying the relationship of 0.272 × TS + 300 is excellent in formability into an actual member, and at the same time has a higher impact energy absorption capacity than other steel materials, so that the impact energy absorption capacity can be increased without increasing the total weight of the member. Can be improved.

As a result of experiments and examinations conducted by the present inventors, for the same level of tensile strength (TS), (σdyn−σs
t) is the amount of solid solution carbon [C] (% by weight) in the retained austenite contained in the steel sheet before being processed into a member and the average Mn equivalent amount of the steel material {Mneq = Mn + (Ni + Cr +)
It was found to change with Cu + Mo) / 2} (wt%). The carbon concentration in the retained austenite is X
It can be experimentally obtained by line analysis or Moessbauer spectroscopy. For example, by X-ray analysis using Mo Kα rays, ferrite (200) plane, (211) plane and austenite (200) plane, (220) plane , Of the Iron and Steel Ins using the integrated reflection intensity of the (311) plane
It can be calculated by the method shown in titute, 206 (1968), p.60. From the results of experiments conducted by the present inventors, a value calculated using Mneq obtained from the solid solution [C] in the retained austenite thus obtained and the substitutional alloying elements added to the steel material ( M = 678-428 × [C] -3
When 3 × Mneq) is −140 or more and less than 70, it is large (σdyn) with respect to the same static tensile strength (TS).
It has been found to exhibit −σst). At this time, M is 7
When it is 0 or more, the retained austenite transforms into hard martensite in the low strain region, which increases the static stress in the low strain region that governs the formability and deteriorates the formability such as shape fixability. As well as (σdyn−
Since the value of σst) is made small, it is not possible to obtain both good moldability and high impact energy absorption ability, so M was set to less than 70. When M is less than −140,
Since the transformation of retained austenite is limited to a high strain region, good formability is obtained (σdyn-σ
Since the effect of increasing st) is lost, the lower limit of M is set to -140.

Manufacturing conditions: When the steel sheet of the present invention is manufactured by cold rolling / annealing after hot rolling, a slab adjusted to have predetermined components is directly sent to the hot rolling step as it is, or once cooled. After that, it is reheated and hot rolled, then pickled, cold rolled,
Then, the product is finished by continuous annealing. At this time, the hot rolling finish temperature is generally set to an Ar ₃ transformation temperature or higher determined by the chemical composition of the steel, but from Ar ₃ to 10 ° C.
It does not deteriorate the properties of the final steel sheet up to a low temperature. In addition, the coiling temperature after cooling is set to be equal to or higher than the bainite transformation start temperature determined by the chemical composition of steel, so that it is possible to avoid increasing the load during cold rolling more than necessary, but when the total reduction ratio of cold rolling is small. Is not limited to this, and the properties of the final steel sheet will not be deteriorated even if wound at a temperature below the bainite transformation temperature of the steel. Further, the total reduction ratio of cold rolling is set from the relationship between the final plate thickness and the cold rolling load, but if it is 40% or more, the properties of the final steel sheet are not deteriorated.

When annealing after cold rolling, the annealing temperatures are determined by the chemical composition of the steel, Ac ₁ and Ac ₃ temperatures (for example, "Steel Material Science": W. C. Leslie, translated by Shigeyasu Koda, Maruzen p273). Expressed 0.1 × (Ac ₃ −A
In the case of less than c ₁ ) + Ac ₁ ° C, the amount of austenite obtained at the annealing temperature is small, so that residual austenite cannot be stably left in the final steel sheet.
The lower limit of the annealing temperature was 0.1 × (Ac ₃ −Ac ₁ ) + Ac ₁ ° C. Further, even if the annealing temperature exceeds Ac ₃ + 50 ° C, the properties of the steel sheet cannot be improved at all, but in order to increase the manufacturing cost, the upper limit of the annealing temperature is set to Ac ₃ +5.
It was set to 0 ° C. The annealing time at this temperature needs to be 10 seconds or more in order to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 3 minutes, not only the effect will be saturated, but also the cost will increase, so 3 minutes was made the upper limit.

The subsequent primary cooling is important for promoting the transformation of austenite to ferrite and concentrating C in the untransformed austenite to stabilize the austenite. If this cooling rate is less than 1 ° C / sec, the production line disadvantages such as lengthening the required production line length and extremely slowing down the production rate occur, so the lower limit of this cooling rate is 1 ° C / sec. And On the other hand, when the cooling rate is higher than 10 ° C./sec, ferrite transformation does not sufficiently occur and it becomes difficult to secure the retained austenite in the final steel sheet, so this was made the upper limit. This primary cooling is 5
When the temperature is lower than 50 ° C., pearlite is generated during cooling, C which is an austenite stabilizing element is wasted, and a sufficient amount of residual austenite cannot be finally obtained. Therefore, the lower limit is 550 ° C. However, the cooling is 67
When the temperature is higher than 0 ° C, the ferrite transformation does not proceed sufficiently. Therefore, the upper limit was 670 ° C.

The rapid cooling of the subsequent secondary cooling is
A cooling rate of at least 10 ° C./second or more is required so that pearlite transformation or precipitation of iron carbide does not occur during cooling. However, since it is difficult to set the cooling rate to more than 200 ° C./sec in terms of facility capacity, the cooling rate range was set to 10 to 200 ° C. The cooling stop temperature of this secondary cooling is 3
When the temperature is 20 ° C. or lower, the properties of the obtained retained austenite are not preferable, and the final attempt is made to obtain (σdyn
In order to reduce the value of −σst), the lower limit was made to exceed 320 ° C. Further, when the secondary cooling stop temperature is higher than 500 ° C, the required amount of retained austenite cannot be obtained, so 500 ° C is set as the upper limit.

In order to stabilize the austenite remaining in the steel sheet at room temperature, it is essential to further increase the carbon concentration in the austenite by transforming part of it into bainite. When the secondary cooling stop temperature is lower than the temperature maintained for the bainite transformation process,
It is heated to the holding temperature. The heating rate at this time is 5 ° C /
Within the range of seconds to 50 ° C./second, the final characteristics are not deteriorated. On the contrary, when the secondary cooling stop temperature is higher than the bainite treatment temperature, the target temperature is set in advance even if the bainite treatment temperature is forcibly cooled at a cooling rate of 5 ° C / sec to 200 ° C / sec. Even if it is directly conveyed to the heated zone, the final properties of the steel sheet are not deteriorated.
On the other hand, when the steel sheet is held at 320 ° C. or lower, the static deformation resistance of the steel sheet increases and the value of (σdyn−σst), which shows both workability and impact energy absorption capacity, is decreased. The lower limit was over 320 ° C. Further, when the temperature is maintained above 500 ° C, a sufficient amount of retained austenite cannot be secured, so the range of the holding temperature exceeds 320 ° C.
It was set to 500 ° C. or less. At this time, more than 320 ℃ ~ 500 ℃
If retained below 15 seconds, the progress of bainite transformation is insufficient, so that the required amount of retained austenite cannot be finally obtained, and if it exceeds 20 minutes, precipitation of iron carbide and pearlite after bainite transformation. Since the transformation occurs and C, which is indispensable for the formation of retained austenite, is wasted and the retained austenite cannot be obtained, the holding time is set to 15 seconds to 20 minutes. Over 320 ℃ ~ 500 ℃ to accelerate bainite transformation
The following holding does not deteriorate the properties of the final steel sheet even if it is held isothermally or if the temperature changes within this temperature range.

[0030]

[Example] [Example 1] 25 kinds of steel materials shown in Table 1 were heated to 1200 ° C, and Ar
Hot rolling was completed at ₃ transformation temperatures or higher, and after cooling, the steel strip wound at the bainite transformation start temperature or higher determined by the chemical composition of each steel was pickled and cold rolled to a thickness of 1.0 mm. Then, from the composition of each steel, Ac ₁ = 723-10.7 × Mn% −16.9.
× Ni% + 29.1 × Si% + 16.9 × Cr, Ac ₃
= 910−203 × (C%) ^{1 /} 2−15.2 × Ni% +
44.7 x Si% + 104 x V% + 31.5 x Mo%-
30 x Mn% -11 x Cr% -20 x Cu% + 700 x
Ac ₁ transformation temperature calculated by P% + 400 × Al% + 400 × Ti% and a temperature calculated from Ac ₃ (Ac ₁ +
Ac ₃ ) / 2 for 90 seconds, cooled to 670 ° C. at 5 ° C./second, cooled to 300 ° C. at 100 ° C./second, reheated, and subjected to bainite transformation treatment at 400 ° C. for 5 minutes and then at room temperature. Pre-deformation of 5% was added by uniaxial tension in the cold rolling direction (L direction) of the cold rolled steel sheet cooled to (L direction) and the direction orthogonal to this (C direction), and 170 ° C. was applied to simulate the baking process.
Table 2 shows the results of examining the dynamic properties of the steel material after the heat treatment for 20 minutes and comparing it with the static properties before pre-deformation. For those having a tensile strength of 588 MPa or more and a steel composition within the range of the present invention, the value shown in the column of * 1 in the table is positive, that is, as intended (σdyn-σst)
Is (−0.272 × TS + 300) or more. [Example 2] The results of investigating the characteristics when the annealing condition, the pre-deformation condition and the heat treatment condition were changed using the steel P2 within the composition range of the present invention shown in Table 1 are shown in Table 3 and Table. 4 shows.
The Ac ₁ and Ac ₃ transformation temperatures of P2 steel were calculated to be 742,848 ° C. After pickling the hot-rolled steel sheet, cold-roll it to a thickness of 1.0 mm,
It was annealed under various annealing conditions.

No. In No. 1, the annealing temperature was outside the range of the present invention, and the required amount of retained austenite was not obtained. In addition, No. In No. 2, the cooling stop temperature of the primary cooling is 500 ° C., which is outside the range of the present invention, so that the retained austenite is prevented from being secured by the pearlite generated during the cooling. In addition, No. In No. 3, since the cooling rate of the secondary cooling is outside the scope of the present invention, the retained austenite is not ensured by the pearlite generated during the cooling. In addition, No. In No. 4, since the cooling stop temperature of the secondary cooling was too low, the amount of martensite formed increased, and retained austenite could not be secured. In addition, No. In No. 7, the secondary cooling rate and the bainite transformation treatment temperature were too high, and retained austenite could not be secured due to the formation of iron carbide. Furthermore, N
o. In No. 9, since the annealing temperature was set higher than necessary,
The coarsening of the structure progresses and the grain size of retained austenite increases, and as a result, a sufficiently large difference between the dynamic deformation resistance and the static deformation resistance cannot be obtained. All other examples are examples of the present invention, and if the annealing conditions are within the scope of the present invention, the form of pre-deformation and work hardening after pre-deformation (BH treatment:
(With heat treatment at 170 ° C for 20 minutes) *
The value in the column 1 is positive, that is, the predetermined increase in deformation resistance (σ
It can be seen that dyn−σst) is obtained. Here, the L direction refers to the same direction as the hot rolling, and the C direction refers to the direction orthogonal to this.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

EFFECTS OF THE INVENTION According to the present invention, it is possible to reliably provide a good workability high strength cold rolled steel sheet for impact absorption at the time of collision, which has a high dynamic deformation resistance and can meet the demands of weight reduction of automobiles and ensuring of collision safety. can do.

[Brief description of drawings]

FIG. 1 is an equivalent strain of 3 to 10% when deformed in a strain velocity range of 5 × 10 ^{2 to} 5 × 10 ³ (1 / s), which is an index of impact energy absorption capacity during collision in the present invention. Mean value of deformation stress σdyn in the range and 5 × 10 ^{−4 to} 5 ×
3-1 when deformed in the strain rate range of 10 ^-3 (1 / s)
It is a figure which shows the relationship between the difference ((sigma) dyn- (sigma) st) of the average value (sigma) st of the deformation stress in the equivalent strain range of 0%, and static material strength.

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-9-241788 (JP, A) JP-A-7-62485 (JP, A) JP-A-7-252592 (JP, A) JP-A-7- 207413 (JP, A) JP-A-1-184226 (JP, A) JP-A 64-79345 (JP, A) International Publication 95/029268 (WO, A1) Miura et al., Development of shock absorbing high-strength steel sheet for automobiles Materia, May 20, 1996, Vol. 35 No. 3, P, 570-572 Itabashi et al., High-speed tensile properties of prestrained rolled steel SN490B for building structures, Proc. Of the Japan Society of Materials Science, May 22, 1997, Vol. 46th, P. 291-292 (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02-8/04 C21D 9/46-9/48

Claims (10)

(57) [Claims] 1. C: 0.04% or more and 0.3% by weight
Hereinafter, one or both of Si and Al are 0.5% or more and 3.0% or less in total, and one or more of Mn, Ni, Cr, Cu, and Mo are 0.5% or more and 3.5% or more in total. % Or less, the balance consisting of Fe and unavoidable impurities, and the microstructure of the finally obtained cold-rolled steel sheet contains ferrite and bainite. Either of them is the main phase and the volume fraction is 3
% Is a composite structure with a third phase containing at least% retained austenite, and the amount of solid solution [C] in the retained austenite and the average Mn equivalent of steel {Mneq = Mn + (Ni + Cr + Cu +
Mo) / 2} determined value (M = 678-428 ×
[C] −33 × Mneq) is −140 or more and less than 70,
It has a tensile strength of 588 MPa or more, and after subjecting the steel material to pre- deformation of more than 0% and 10% or less with equivalent strain, 5 × 10 ²
Average value σ of deformation stress in an equivalent strain range of 3 to 10% when deformed in a strain rate range of ˜5 × 10 ³ (1 / s)
dyn (MPa) and 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s)
The difference of the average value σst (MPa) of the deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 5 is
Maximum stress TS (MP in a static tensile test measured in a strain rate range of × 10 ^{-4 to} 5 × 10 ^-3 (1 / s))
a) expressed by (adyn−σst) ≧ −
A good workability, high strength cold rolled steel sheet having high dynamic deformation resistance for impact shock absorption, which satisfies 0.272 × TS + 300. 2. The impact-absorbing shock having high dynamic deformation resistance according to claim 1, further comprising one or more of Nb, Ti and V in a total amount of 0.3% by weight or less . Good workability High strength cold rolled steel sheet. 3. The good workable high strength cold rolled steel sheet for impact shock absorption having a high dynamic deformation resistance according to claim 1 or 2, further comprising P in an amount of 0.2% by weight or less. 4. Better content of 0.01% by weight or less , good workability and high strength for impact shock absorption with high dynamic deformation resistance according to any one of claims 1 to 3. Cold rolled steel sheet. 5. The residual austenite volume fraction of the steel material after pre-deformation of more than 0% and 10% or more is more than 2.5%, and the residual austenite volume fraction before pre-deformation and after pre-deformation. The ratio of residual austenite volume fraction of is 0.4 or more, good workability and high strength for impact absorption at impact having high dynamic deformation resistance according to any one of claims 1 to 4. Cold rolled steel sheet. 6. The ratio of the average grain size of retained austenite in the microstructure of the finally obtained cold rolled steel sheet to the average grain size of ferrite or bainite as the main phase is 0.6 or less. The high workability high strength cold rolled steel sheet for impact shock absorption having high dynamic deformation resistance according to any one of claims 1 to 5. 7. C: 0.04% or more and 0.3% by weight%
Hereinafter, one or both of Si and Al are 0.5% or more and 3.0% or less in total, and one or more of Mn, Ni, Cr, Cu, and Mo are 0.5% or more and 3.5% or more in total. % Or less, and the balance being Fe and inevitable impurities, the cast slab is sent directly to the hot rolling process as it is, or once cooled and then reheated, the hot rolled steel sheet rolled by hot rolling is pickled. When it is post-cold rolled and annealed in a continuous annealing step to obtain a final product, it is annealed at a temperature of 0.1 × (Ac ₃ −Ac ₁ ) + Ac ₁ ° C. or more and Ac ₃ + 50 ° C. or less for 10 seconds to 3 minutes. After that, at a primary cooling rate of 1 to 10 ° C./sec, 550 to 670 ° C.
Cooling to the primary cooling stop temperature in the range of 10
~ 320 ℃ / 500 at secondary cooling rate of 200 ℃ / sec 500
After cooling to the secondary cooling stop temperature below ℃ 5
Hold in the temperature range of 00 ° C. or lower for 15 seconds to 20 minutes, cool to room temperature, the microstructure of the finally obtained cold-rolled steel sheet contains ferrite and bainite, one of which is the main phase, and the volume fraction is It is a composite structure with a third phase containing 3% or more of retained austenite, and has a tensile strength of 588 MPa or more.
Of the solid solution [C] in the retained austenite and the average Mn equivalent of steel {Mneq = Mn + (Ni + Cr + Cu
+ Mo) / 2} (M = 678-428)
X [C] -33 x Mneq) is 70 or more and 180 or less,
Equivalent strain of 3-10% when deformed in a strain rate range of 5 × 10 ^{2 to} 5 × 10 ³ (1 / s) after subjecting the steel material to pre-deformation of more than 0% and 10% or less with equivalent strain Average value of deformation stress σdyn (MPa) in the range and 5 × 10 ^{−4 to} 5 ×
3-1 when deformed in the strain rate range of 10 ^-3 (1 / s)
Average value of deformation stress σst in the 0% equivalent strain range
The expression (σdyn) represented by the maximum stress TS (MPa) in the static tensile test in which the difference in (MPa) is measured in the strain rate range of 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s).
This to satisfy the -σst) ≧ -0.272 × TS + 300
Impact absorption during collision with high dynamic deformation resistance characterized by
Excessive workability High strength cold rolled steel sheet manufacturing method. 8. The impact-absorbing shock having high dynamic deformation resistance according to claim 7, further comprising one or more of Nb, Ti and V in a total amount of 0.3% by weight or less . Good workability High strength cold rolled steel sheet manufacturing method. 9. The method for producing a good workability high-strength cold-rolled steel sheet for impact shock absorption having high dynamic deformation resistance according to claim 7, further comprising P in an amount of 0.2% by weight or less. . 10. A good workability and high strength for impact absorption at impact having a high dynamic deformation resistance according to claim 7, further comprising 0.01% by weight or less of B. Manufacturing method of cold rolled steel sheet.