JP2002060898A - High strength steel sheet for automotive room structural parts, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet which is used for automotive room structural parts and hardly causes stretch-flange rupture at press working and can suppress the deformation of the room at collision and also to provide its manufacturing method. SOLUTION: The steel sheet has a composition consisting of, by weight, 0.05-0.17% C, 0.5-1.5% Si, 0.8-2.4% Mn, <=0.02% P, <=0.004% S, 0.01-0.1% Al, <=0.006% N, 0.0005-0.004% Ca and the balance Fe with inevitable impurities and has a structure containing >=30% ferrite by volume fraction, >=10% bainite by volume fraction and <6%, in total, of martensite and austenite by volume fraction. In the steel sheet: average grain size of ferrite is <=22 μm; the volume fraction of carbides of >=0.1 μm circle-equivalent radius is <=0.1%;, stretch-flange formability is >=80% by bore expanding ratio; upper yield stress at 500/s strain rate is >=1.5 times the quasi-static tensile strength; and strength is >=490 MPa.

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 for automobile passenger compartment structural parts and a method for producing the same. The high-strength steel sheet according to the present invention is processed into a vehicle cabin structural part through press forming, and is used to protect an occupant in a passenger cabin during a vehicle collision. In addition, from the viewpoint of press workability and rust prevention performance, a steel sheet whose surface is subjected to a treatment such as hot-dip plating, electroplating, an inorganic film, and an organic film is also included in the present invention.

[0002]

2. Description of the Related Art High-strength steel sheets of 490 MPa class or higher, which have high flow stress when plastically deformed at high speed, have been developed in order to enhance the safety of occupants in the event of collision of an automobile body.
These target parts such as front side members that absorb plastic energy due to large plastic deformation at the time of collision. Absorbed energy up to a relatively large strain of about 5% to 10% is particularly important during high-speed deformation. It is important to be big. Such a steel plate is disclosed in JP-A-09-111396 and JP-A-09-29.
No. 6247 discloses a composite structure steel containing martensite. Japanese Patent Application Laid-Open No. H10-158735 discloses a composite structure steel sheet containing retained austenite, and Japanese Patent Application Laid-Open No. H11-100641 discloses a second phase satisfying a certain relationship with the hardness of ferrite (any of martensite, austenite and bainite). A composite structure steel sheet comprising: Further, Japanese Patent Application Laid-Open No. 10-259448 discloses that a composite structure steel sheet comprising bainite and ferrite increases absorption energy in a high-speed tensile test. The feature of these steel sheets is that they use martensite, austenite, and bainite as a second phase, which are originally contained in the steel sheet, and utilize the strain rate dependence of the absorbed energy that is developed because they are harder than the matrix. Is what you are doing. While these conventional steel sheets focus on the absorbed energy during high-speed deformation, the combination of a hard second phase and a soft matrix has inferior stretch flangeability, as is generally known for low-yield-ratio dual-phase steels. It is not suitable for parts with stretch flange forming during press forming, such as cross members and center pillar inners. Further, in a passenger compartment structural component in which large plastic deformation is not allowed to occur at the time of a collision, a conventional steel plate whose main purpose is to enhance plastic deformation absorption energy has a small effect of securing occupant safety.

On the other hand, steel sheets having excellent stretch flangeability are intended for undercarriage parts, wheels, rims and the like even for automobiles, and cannot satisfy the collision safety required for passenger compartment structural parts.

[0004]

As the collision safety of automobiles is being developed, as described above, the conventional steel sheet of the composite structure type for the front side member has been applied to the cabin structure such as the cross member and the center pillar inner. It turned out to be unsuitable for parts. Therefore, development of a high-strength steel sheet for passenger compartment structural parts having high stretch flangeability and high deformation resistance at the time of collision has been considered as an issue.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors first studied what mechanical properties are important for the material for passenger compartment structural parts. as a result,
The workability requires stretch flangeability in addition to elongation, and the safety of the cabin is irrelevant to the absorbed energy due to plastic deformation, and it is important that the deformation resistance to the input maximum stress is important. Strain rate 50
The upper yield point stress in the high strain rate range of about 0 / s is the most important material factor. Even if the material does not show an upper yield point at a low strain rate, it is shown in FIG. 1 at a high strain rate of about 500 / s. The sharp upper yield point is observed as shown in the figure, and the degree of increase of the upper yield point stress due to the strain rate differs depending on the material.The value is relatively small in a composite structure steel with a low yield ratio, and the steel specification is static. It has been found that the ratio of the upper yield point stress to the quasi-static tensile strength at a strain rate of 500 / s needs to be 1.5 or more because the tensile strength is the standard. Furthermore, from experiments using steel sheets having various components and various microstructures, it was found that, when the volume fraction of ferrite, bainite, martensite and retained austenite, and the grain size of ferrite are within a certain range, the above materials We found that the requirements were satisfied. They also found that the form of carbide is an important factor for achieving both stretch flangeability and upper yield point stress at high strain rates. Further, when the penetration type solid solution element is fixed to the dislocation and hardened during baking of the paint, the strength increase is large even in the high strain rate range, and therefore, the strength at the strain rate of 500 / s is not increased without greatly increasing the quasi-static tensile strength. They found that the yield point stress could be raised, and found that even if dislocations could not be introduced by press forming, the dislocations introduced in the temper rolling step were effective.

[0006] The present invention is based on such new knowledge,
This is a new steel plate constructed based on the concept not found in the conventional steel plate for collision safety, and the gist is as follows.

(1) The tensile strength is 490 MPa or more, the hole expansion ratio is 80% or more, and the upper yield point at a strain rate of 500 / s is 1.5 times or more the quasi-static tensile strength. High-strength steel sheet for automobile passenger compartment structural parts.

(2) In the above (1), C: 0.05 to 0.17%, Si: 0.5 to 1.5%, Mn: 0.8 to 2.4%, P : 0.02% or less, S: 0.004% or less, Al: 0.01 to 0.1%, N: 0.006% or less, with the balance being Fe and unavoidable impurities. High-strength steel sheet for automotive cabin structural parts.

(3) In the above (1) or (2),
When the ferrite contains at least 30% ferrite by volume fraction, contains at least 10% bainite by volume fraction, the combined volume fraction of martensite and austenite is less than 6%, and the average crystal grain size of ferrite is 22 μm or less. A high-strength steel sheet for automobile passenger compartment structural parts, wherein the volume fraction of carbide having a circle equivalent radius of 0.1 μm or more is 0.1% or less.

(4) In the above (1) or (2),
A ferrite having a volume fraction of at least 30%, a bainite having a volume fraction of at least 10%, a martensite and austenite volume fraction of less than 6%, and an average ferrite grain size of at least 10 μm; A high-strength steel sheet for automobile passenger compartment structural parts, wherein the volume fraction of carbide having a circle equivalent radius of 0.1 μm or more is 0.1% or less.

(5) In the above (1) or (2),
It contains 30% or more of ferrite by volume fraction, contains 10% or more of bainite by volume fraction, the combined volume fraction of martensite and austenite is less than 6%, and the average grain size of ferrite is 10 μm to 22 μm. A high-strength steel sheet for automobile passenger compartment structural parts, wherein a volume fraction of carbide having a circle equivalent radius of 0.1 μm or more is 0.1% or less.

(6) A high-strength steel sheet for automobile passenger compartment structural parts according to the above (2), (3), (4) or (5), which contains 0.0005 to 0.004% of Ca.

(7) A steel slab having the chemical composition described in any of (2) and (6) above is heated at a temperature of (Ac ₁ transformation point + 50) ° C. or more and (Ar ₃ transformation point + 50) ° C. or less. After finish rolling, average cooling rate 5 after finishing rolling
At a speed of 0 ° C./s or more, T = 650−450 × [% C] + 40 × [% Si] −6
Cooling is performed to a range of T ° C or lower (T-60) ° C or higher, which is calculated by 0 × [% Mn] + 470 × [% P], and then, after air cooling, over 350 to 500 ° C.
Characterized in that it is subjected to a temper rolling of 0.6 to 3% in a cold state, and further includes a ferrite of 30% or more in volume fraction and a bainite of 10% or more in volume fraction,
The combined volume fraction of martensite and austenite is less than 6%, the average crystal grain size of ferrite is 22 μm or less, and the volume fraction of carbide having a circle equivalent radius of 0.1 μm or more is 0.1% or less, High-strength steel sheet for automobile passenger compartment structural parts having a tensile strength of 490 MPa or more, a hole expansion ratio of 80% or more, and an upper yield point at a strain rate of 500 / s of 1.5 times or more the quasi-static tensile strength. Manufacturing method.

(8) A hot-rolled steel sheet having a composition consisting of the chemical component according to any of (2) or (6) above is directly or cold-rolled to obtain (Ac ₁ transformation point +50)
C. (Ar ₃ transformation point +50) ° C. or lower for 30 s or more, and from the temperature range at an average cooling rate of 0.5 to 10 ° C./s, U = 723 + 30 × [% Si] −10 × [% Mn Cooling is performed to a temperature range of not more than U ° C (U-170) ° C or more, and then cooled to a range of more than 350 to 500 ° C at an average cooling rate of 10 ° C / s or more. Holds 70 s or more, and further performs cold pass tempering of 0.6 to 3%, including ferrite with a volume fraction of 30% or more, and containing bainite with a volume fraction of 10% or more. , The combined volume fraction of martensite and austenite is less than 6%, and the average grain size of ferrite is 2%.
2 μm or less, carbide having a circle equivalent radius of 0.1 μm or more has a volume fraction of 0.1% or less and a tensile strength of 490 M
Pa or higher, hole expansion ratio is 80% or more, strain rate 50
A method for producing a high-strength steel sheet for automobile passenger compartment structural parts, wherein the upper yield point at 0 / s is 1.5 times or more of the quasi-static tensile strength.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. First, the reasons for limiting the numerical values of C, Si, Mn, P, S, Al, N, and Ca will be described.

C is an element indispensable for enhancing the strength of the steel sheet by bainite. If C is less than 0.05%, sufficient strength cannot be obtained. However, if it exceeds 0.17%, the spot weldability deteriorates. Therefore, C is 0.0
5 to 0.17%.

Si is the most important element for improving the stretch flangeability. If it is less than 0.5%, the stretch flangeability is poor.
However, if it exceeds 1.5%, the weldability is deteriorated, and the upper yield point stress at a high strain rate is lower than the quasi-static tensile strength, for unknown reasons. Therefore Si
Was set to 0.5 to 1.5%.

Mn is an element necessary for increasing the strength and increasing the upper yield point at a high strain rate, and must be contained at 0.8% or more. Mn is effective in suppressing coarsening of ferrite grain size even at a low cooling rate. If it is less than 0.8%, an extremely high cooling rate is required to reduce the grain size of the ferrite. However, if it exceeds 2.4%, the performance becomes unstable due to the non-uniformity of the Mn distribution in the steel. When the ferrite content is 30% or more, as the grain size of ferrite increases, the strain rate dependence of the upper yield point stress of steel increases, and the upper yield point stress at high strain rates increases. The lower, the greater. Since Mn suppresses the formation of coarse carbides even at a relatively low cooling rate, a large ferrite grain size can be secured without impairing stretch flangeability.

P is generally contained in steel as an unavoidable impurity, but if its amount exceeds 0.02%, the spot weldability is greatly deteriorated, and the toughness and cold rollability during impact deformation are also deteriorated.

S is generally contained in steel as an unavoidable impurity, but if its amount exceeds 0.004%, sulfide inclusions adversely affect the stretch flangeability.

Al is required to be 0.01% or more as a deoxidizing element, but if it is 0.1% or more, alumina inclusions become remarkable and the stretch flangeability is deteriorated.

N is also generally contained in steel as an unavoidable impurity, but if its amount exceeds 0.006%, the stretch flangeability deteriorates, so this is set as the upper limit.

Ca must be added in an amount of 0.0005% or more in order to control the form of the sulfide-based inclusions and render the harmlessness to the stretch flangeability. However, if it exceeds 0.004%, the adverse effect of Ca-based inclusions becomes a problem.

Next, the characteristics of the microstructure of the high-strength steel sheet according to the present invention will be described.

Ferrite is a very important phase in order to secure the strain rate dependence of the upper yield point stress and to increase the upper yield point stress at a high strain rate. The yield ratio of the composite structure steel decreases as the strength difference between the ferrite and the second phase increases, and the upper yield point does not sufficiently increase even under a high strain rate.
Generally, the smaller the grain size of ferrite, the higher the strength. However, at a high strain rate of 500 / s or more, the strain rate dependency is also important. In addition, ferrite increases strain rate dependence as compared with the second phase. Its volume fraction is 30%
If it is less than 10, the degree of increase of the upper yield strength with the increase of the strain rate is small. Furthermore, the volume fraction of ferrite is 30
%, The decrease in the strain rate dependence of the upper yield point stress due to the other second phase cannot be sufficiently compensated. Therefore, ferrite needs to be 30% or more in volume fraction. In particular, in order for the strain rate dependence of ferrite to be sufficient, the average particle size is desirably 10 μm or more. In addition, the ferrite strength is sufficient, the dynamic upper yield strength is high, and the quasi-static tensile strength is increased to satisfy the practical strength other than the impact strength required as a part. The particle size is desirably 22 μm or less.

Bainite is important for securing quasi-static strength without impairing stretch flangeability. If the volume fraction is less than 10%, sufficient strength cannot be obtained.

Martensite and austenite reduce the stretch flangeability and the upper yield point stress when the volume fraction is 6% or more. Further, martensite is not preferred because it suppresses a dynamic increase in yield strength.

[0028] Carbides are effective in increasing the yield strength, but the effect is recognized even at a high strain rate. However, if the carbide having a circle equivalent radius of 0.1 μm or more has a volume fraction of 0.1% or more, the stretch flangeability decreases.

Next, the reasons for limiting the manufacturing conditions will be described.
First, the manufacturing conditions of the hot-rolled steel sheet will be described.

The finishing temperature of the hot rolling is determined by combining the components within the scope of the present invention with the subsequent cooling conditions to obtain the required amounts of bainite and ferrite, and finally reduce the average grain size of the ferrite to 22 μm or less. To do that. (Ac ₁ transformation point + 50) 10% or less at below ℃
The above bainite cannot be obtained, or the obtained ferrite has a grain size of 22 μm or more. (Ar ₃ transformation point +50)
Above ° C, the finally obtained ferrite has a volume fraction of 3
Less than 0%.

The average cooling rate after finishing rolling is 20 ° C.
/ S or more. At 20 ° C./s or less, the austenite ferrite transformation proceeds during cooling, and the amount of bainite finally obtained is 10% or less. At 100 ° C./s or more, this effect is saturated.

The end temperature of the rapid cooling is T = 650-450 × [% C] + 40 × [% Si] -6
It is necessary to have a temperature calculated by the following equation: 0 × [% Mn] + 470 × [% P] T ° C. or less (T−60) ° C. or more. At a temperature exceeding T ° C., the stretch flangeability decreases due to the formation of pearlite. Below (T-60) ° C, the quasi-static strength becomes unstable.

It is necessary to perform air cooling from the rapid cooling end point temperature and to adjust the winding temperature to more than 350 to 500 ° C. This is for securing 10% or more of bainite by cooling in the state of a coil after winding from air cooling to winding and avoiding generation of other phases. At a winding temperature of 350 ° C. or lower, martensite is undesirably mixed due to increased mixing of martensite. On the other hand, when the temperature exceeds 500 ° C., not only bainite is not obtained, but also the stretch flangeability is deteriorated due to the formation of 0.1% or more of carbides of 0.1 μm or more.

Further, it is necessary to perform cold-rolling of 0.6% to 3% in the cold state. This not only prevents the stretcher strain but also the C and the solid solution in the steel during the baking treatment. The penetration of N atoms fixes the dislocations introduced by temper rolling, thereby increasing the upper yield point stress at a strain rate of 500 / s while minimizing the increase in static tensile strength. It was aimed. It has been found that the steel sheet produced in the above process contains supersaturated penetrating atoms at room temperature without any special measures for cooling after winding and has a large amount of paint bake hardening. Dislocations are also introduced during press working, but in the case of cross members or center pillar inners mainly formed by bending, there are a wide range of parts that are not deformed. Introduce and ensure the performance. 0.6%
If it is less than 30, a sufficient effect is not recognized. If it is 3% or more, workability including stretch flangeability deteriorates.

Next, conditions for manufacturing a steel sheet of the present invention by applying a material adjusted to a desired thickness to a continuous annealing line by arbitrary hot rolling or cold rolling once will be described. The temperature condition is slightly different from the temperature condition in the above-mentioned hot rolling, but this is because cooling can be rapidly performed immediately before holding the bainite transformation temperature at more than 350 to 500 ° C.

The annealing temperature is set so that the required amount of bainite and ferrite can be obtained by combining the components in the range of the present invention with the subsequent cooling conditions, and the average grain size of the ferrite is finally reduced to 22 μm or less. At (Ac ₁ transformation point +50) ° C. or lower, 10% or more of bainite is not finally obtained, or the grain size of the obtained ferrite is 22 μm.
That is all. Above (Ar ₃ transformation point +50) ° C., the volume fraction of the finally obtained ferrite is less than 30%.
If the annealing temperature is less than 30 s, a sufficient amount of austenite is not generated even in this temperature range, so that bainite finally obtained is less than 10%.

The first cooling is performed at an average cooling rate of 0.5 to 10 ° C./s. The temperature calculated by U = 723 + 30 × [% Si] −10 × [% Mn] is not higher than U ° C. and is not lower than (U-170) ° C. , And the subsequent cooling must be performed at an average cooling rate of 10 ° C./s or more to a range of more than 350 to 500 ° C. If the initial cooling rate is less than 0.5 ° C./s, the productivity is reduced, so this is set as the lower limit. If it exceeds 10 ° C./s, the C concentration in austenite is low, and martensite is formed during the next rapid cooling, which lowers the yield strength and stretch flangeability. If the first cooling end temperature exceeds U ° C., a sufficient amount of ferrite cannot be secured. If the temperature is lower than (U-170) C, a large amount of coarse carbides is generated during the cooling. If the subsequent cooling rate is less than 10 ° C./s, austenite is transformed into pearlite, and not only the quasi-static strength is reduced, but also the stretch flangeability is deteriorated.

Thereafter, 70 to 350 ° C. to 500 ° C.
More than 10% of bainite is to be kept,
This is to avoid formation of other phases. If the temperature is lower than 350 ° C., a large amount of austenite or martensite is mixed.
On the other hand, when the temperature exceeds 500 ° C., not only bainite is not obtained, but also carbides having a diameter of 0.1 μm or more are generated in an amount of 0.1% or more. If the holding time is 70 s or less, the bainite transformation is insufficient, so that when cooling to room temperature is completed, martensite will eventually be increased and included.

Further, it is necessary to perform cold-rolling of 0.6% to 3% in the cold state, for the same reason as explained in the above-mentioned production conditions in the hot rolling step.

[0040]

EXAMPLES Steels having the components shown in Table 1 were hot-rolled by the method shown in Table 2, and once cooled to room temperature, were subjected to temper rolling with strains shown in Table 2. The plate thickness was 1.8 mm. In addition, steel No. 4 in Table 1 was subjected to hot rolling by the hot rolling method of E in Table 2 for 1.
A steel sheet cold-rolled to 8 mm was prepared from 8 mm, and subjected to heat treatment and temper rolling under the conditions shown in Table 3. These samples were subjected to a quasi-static tensile test and a hole expanding test. Ferrite particle diameter Df (μm), ferrite volume fraction Vf, bainite volume fraction Vb, martensite
The austenite volume fraction Vm was measured from an SEM photograph. TE for measuring the volume fraction of carbides of 0.1 μm or more
The high magnification and the low magnification of the M photograph were used in combination, and the average value was measured for the entire steel. The quasi-static tensile test was performed using a JIS No. 5 test piece at a strain rate of 0.003 / s, and the tensile strength TS
(MPa) was measured. In the hole expansion test, a punched hole having a diameter of 10 mm is pushed and expanded with a conical punch having a vertex angle of 60 °, and first, a hole diameter d (mm) when a crack penetrates in a thickness direction is measured, and a hole expansion ratio λ (%) is measured. = 100 × (d−10) / 10
Was evaluated. A high-speed tensile test at a strain rate of 500 / s was performed on a material that had been subjected to a heat treatment equivalent to paint baking at 170 ° C. for 20 minutes, and a stress value DYPU (MP
a) was measured.

The required hole expansion ratio depends on the shape of the part and the press forming method. If λ is 80% or more, the stretch flange does not break or the stretch flange break can be avoided by slightly changing the blank shape. There are many cross members and center pillar inners. DYP when λ is 80% or more
490MPa tensile strength satisfying U / TS of 1.5 or more
The above high-strength steel sheets are shown in Table 4 as 01A, 02A, 03
B, 04B, 05B, and 04M. These are within the scope of the present invention, but the other steels are TS or λ
Or, DYPU / TS is not satisfied. As described above, according to the present invention, it is possible to provide a high-strength steel sheet for a vehicle cabin structural component such as a cross member or a center pillar inner.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

[Table 4]

[0046]

As described above in detail, according to the present invention, a high-strength steel sheet for an automobile passenger compartment structural part, which can prevent the stretch flange from breaking during press working and satisfy the collision performance as a part. be able to. This greatly contributes to automobile productivity, collision safety, and body weight reduction,
It has an extremely large industrial effect.

[Brief description of the drawings]

FIG. 1 is a graph showing a stress value (DYPU) at an upper yield point observed in a stress (σ) -strain (ε) curve measured by a high-speed tensile test at a strain rate of 500 / s.

Continuing from the front page (72) Inventor Eishin Murasa 1 Kimitsu, Kimitsu City F-term in the Nippon Steel Corporation Kimitsu Works (reference) 4K037 EA01 EA05 EA06 EA09 EA15 EA16 EA18 EA23 EA25 EA27 EA28 EB05 EB06 EB09 EB12 FC04 FD04 FD05 FE01 FF03 FJ06 FK02 FK03 FK08 FM02 GA05 JA06 JA07

Claims (8)

[Claims] 1. The tensile strength is 490 MPa or more, the hole expansion ratio is 80% or more, and the upper yield point at a strain rate of 500 / s is 1.5 times or more the quasi-static tensile strength. High-strength steel sheet for automobile passenger compartment structural parts. 2. The method according to claim 1, wherein C: 0.05 to 0.17%, Si: 0.5 to 1.5%, Mn: 0.8 to 2.4%, P: 0 in mass%. 0.02% or less, S: 0.004% or less, Al: 0.01 to 0.1%, N: 0.006% or less, with the balance being Fe and inevitable impurities. High strength steel sheet for structural parts. 3. A ferrite having a volume fraction of at least 30%, a bainite having a volume fraction of at least 10%, and a combined volume fraction of martensite and austenite of 6%.
Less than, the average crystal grain size of the ferrite is 22 μm or less,
The volume fraction of carbide having a circle equivalent radius of 0.1 μm or more is 0.1 μm.
3. The method according to claim 1, wherein the amount is 1% or less.
A high-strength steel sheet for a vehicle cabin structural part according to claim 1. 4. A ferrite having a volume fraction of at least 30%, a bainite having a volume fraction of at least 10%, and a combined volume fraction of martensite and austenite of 6%.
Less than, the average crystal grain size of the ferrite is 10 μm or more,
The volume fraction of carbide having a circle equivalent radius of 0.1 μm or more is 0.1 μm.
3. The method according to claim 1, wherein the amount is 1% or less.
A high-strength steel sheet for a vehicle cabin structural part according to claim 1. 5. A ferrite having a volume fraction of not less than 30%, a bainite having a volume fraction of not less than 10%, and a combined volume fraction of martensite and austenite of 6%.
Less than 10 μm to 22 μm
3. The high-strength steel sheet for automobile passenger compartment structural parts according to claim 1, wherein the volume fraction of carbide having a circle equivalent radius of 0.1 μm or more in m is 0.1% or less. 4. 6. The high-strength steel sheet for automobile passenger compartment structural parts according to claim 2, wherein Ca: 0.0005 to 0.004% is contained. 7. A steel slab having the chemical composition according to claim 2 or ₃ is subjected to finish rolling at a temperature of (Ac ₁ transformation point + 50) ° C. or more and (Ar ₃ transformation point + 50) ° C. or less, and finish rolling is performed. T = 650−450 × [% C] + 40 × [% Si] −6 at an average cooling rate of 50 ° C./s or more from the end.
Cooling is performed to a range of T ° C or lower (T-60) ° C or higher, which is calculated by 0 × [% Mn] + 470 × [% P], and then, after air cooling, over 350 to 500 ° C.
Characterized in that it is subjected to a temper rolling of 0.6 to 3% in a cold state, and further includes a ferrite of 30% or more in volume fraction and a bainite of 10% or more in volume fraction,
The combined volume fraction of martensite and austenite is less than 6%, the average crystal grain size of ferrite is 22 μm or less, and the volume fraction of carbide having a circle equivalent radius of 0.1 μm or more is 0.1% or less, High-strength steel sheet for automobile passenger compartment structural parts having a tensile strength of 490 MPa or more, a hole expansion ratio of 80% or more, and an upper yield point at a strain rate of 500 / s of 1.5 times or more the quasi-static tensile strength. Manufacturing method. 8. A hot-rolled steel sheet having a composition comprising the chemical components according to claim 2 or 6 as it is or subjected to cold rolling, and is (Ac ₁ transformation point + 50) ° C. or more (Ar ₃ transformation point + 5).
0) Anneal at a temperature of not more than 30 ° C. for not less than 30 s, and from the temperature range at an average cooling rate of 0.5 to 10 ° C./s, U = 723 + 30 × [% Si] −10 × [% Mn] ℃ or lower (U-170) ℃ or more, and then cooled to a range of more than 350 to 500 ° C. at an average cooling rate of 10 ° C./s or more, and kept at that temperature for 70 s or more. Characterized in that temper rolling of 0.6 to 3% is performed between them, containing ferrite of volume fraction of 30% or more, containing bainite of volume fraction of 10% or more, and combining martensite and austenite. Is less than 6% and the average grain size of ferrite is 2%.
2 μm or less, carbide having a circle equivalent radius of 0.1 μm or more has a volume fraction of 0.1% or less and a tensile strength of 490 M
Pa or higher, hole expansion ratio is 80% or more, strain rate 50
A method for producing a high-strength steel sheet for automobile passenger compartment structural parts, wherein the upper yield point at 0 / s is 1.5 times or more of the quasi-static tensile strength.