JP2002069574A - Low yield ratio and high strength cold rolled steel sheet excellent in pore expansibility, and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio and high strength steel sheet whose elongation in a tensile test and hole expansibility are made compatible with each other, and to provide its production method. SOLUTION: The steel controlled to be (%Ti)/(%S)>=5 (wherein, %S and %Ti respectively denote S and Ti contents) is hot-rolled at the finish outlet side temperature of (900+50×%Si) deg.C or lower and is then annealed at 700 to 900 deg.C being a two phase coexistent temperature region of ferrite and austenite to form a metallic structure in which the total volume ratio of martensite and retained austenite is >=6%, and the hard phase structure of martensite, retained austenite and bainite are contained by fixed ratios in accordance with the amounts of precipitation strengthening elements such as Ti, Nb, Mo and V to be added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-yield-ratio high-strength steel sheet and a plated steel sheet having excellent hole expandability and a method for producing the same. The high-strength steel sheet according to the present invention is used by press working in applications such as automobile parts, home electric appliances, and construction, and is subjected to hot-dip galvanizing or electro-galvanizing to improve rust prevention. In addition, a steel sheet having a surface treated with a metal oxide film or an organic film for further improvement or a steel sheet having an upper layer provided with iron plating for improving press formability is included.

[0002]

The an automobile body have been required along with lighter, such as secure functions such as protecting the occupant realized by improved fuel economy and reduction of exhaust gas including CO ₂ in the event of a collision. In response to this, we used solid solution strengthening and composite structure strengthening in which martensite and bainite are finely dispersed in a ferrite matrix, which are generally considered as strengthening mechanisms that strengthen steel sheets without deteriorating press workability. High strength steel plates have been developed. Further, in recent years, steel sheets have been developed which attempt to improve elongation in a tensile test by utilizing transformation-induced plasticity of retained austenite as disclosed in Japanese Patent No. 2017320 and Japanese Patent No. 2545316. The quality of not only depends on the elongation of the tensile test, but also depends on the quality of the ductility of the sheared end face, which is called the hole-expanding property in a member with severe stretch flange processing, so the hole-expanding property is not always good. The use of high-strength steel sheets has been limited.

[0003] As a method of improving the hole expandability of a steel sheet,
It has been proposed to reduce S, which leads to an increase in expanded inclusions such as MnS, and to avoid the mixing of martensite to break sharp voids formed at phase boundaries where the deformation stress differs greatly. However, extremely reducing S in steel not only causes a remarkable increase in cost, but also cannot be realized industrially easily. In addition, if inclusions containing S cannot be seen, adverse effects of other inclusions are evident. This is not realistic. Further, even if it is attempted to increase the strength by avoiding the presence of martensite, in order to obtain a high strength of 590 MPa or more, a metal structure mainly composed of bainite, Ti, Nb, M
Although elements such as o and V must be added to enhance precipitation strengthening, in general, in these methods, the ratio of yield strength to tensile strength, that is, the yield ratio is high, which is not preferable from the viewpoint of press workability. Ti, Nb, Mo,
Since elements such as V increase the recrystallization temperature, it is difficult for cold rolled steel sheets to fully utilize the precipitation strengthening in the recrystallized state.

[0004]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a high-strength composite microstructure steel sheet in which martensite, retained austenite and bainite are finely dispersed in a ferrite matrix to improve the press workability. It is an object of the present invention to achieve both elongation and hole expandability in a tensile test so that a member with strict stretch flange processing can be pressed while securing the same.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have developed a metal structure in which sharp voids are not formed at the phase boundary between ferrite and martensite on the sheared end face. Can be realized by strengthening with fine precipitates containing elements such as fine Ti, Nb, Mo, and V, and reducing the difference in deformation stress between adjacent structures. After the adjustment, the steels to which the precipitation strengthening elements such as Ti, Nb, Mo, and V were added, and the hot rolling temperature and the annealing temperature conditions after the cold rolling were intensively studied. As a result,% S,% Ti When the content is adjusted to S and Ti, respectively, the steel whose composition is adjusted so that (% Ti) / (% S) ≧ 5 is obtained. When the content of% Si is set to Si, the finishing temperature is set to (900 + 50 ×% i) After performing hot rolling at a temperature of not more than ° C, annealing is performed in a temperature range of coexistence of ferrite and austenite at 700 to 900 ° C, so that the martensite and the retained austenite have a total volume ratio of 6% or more, and If the hard phase structure of the site, retained austenite and bainite forms a metal structure that is contained at a certain ratio corresponding to the added amount of the precipitation strengthening element such as Ti, Nb, Mo, and V, the required strength is ensured and the strength is lowered. In the yield ratio,
It has been found that both elongation and hole expandability in a tensile test can be achieved.

[0006] The present invention is an unprecedented and completely new low-yield-ratio high-strength steel sheet having an excellent hole-expansion property based on such ideas and new findings. is there.

(1) In mass%, C: 0.04 to 0.1
4%, Si: 0.4 to 2.2%, Mn: 1.2 to 2.4
%, P: 0.02% or less, S: 0.01% or less, Al:
0.002-0.5%, Ti: 0.005-0.1%,
N: not more than 0.006%, and when% S and% Ti are respectively S and Ti contents, (% Ti) / (%
S) A low-yield-ratio high-strength cold-rolled steel sheet which satisfies ≧ 5 and is composed of a balance of Fe and inevitable impurities, and is excellent in hole expandability.

(2) C: 0.04 to 0.1 in mass%
4%, Si: 0.4 to 2.2%, Mn: 1.2 to 2.4
%, P: 0.02% or less, S: 0.01% or less, Al:
0.002-0.5%, Ti: 0.005-0.1%,
N: 0.006% or less and further contains at least one of Nb, Mo, and V in an amount of 0.005 to 0.1%, and further contains% S,
% Ti as S and Ti content respectively (% Ti)
A low-yield-ratio high-strength cold-rolled steel sheet that satisfies / (% S) ≧ 5 and is composed of a balance of Fe and inevitable impurities, and is excellent in hole expandability.

(3) B: 0.0010% by mass
The low-yield-ratio high-strength cold-rolled steel sheet excellent in hole expandability according to the above (1) or (2), containing 0.003%.

(4) The above (1), (2) or (3)
Low yield ratio high strength plated steel sheet with excellent hole expandability described in 1.

(5) The above (1), (2) or (3)
Wherein the total volume fraction of martensite and retained austenite in the metal structure is 6% or more, and the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti,
The addition amounts of Nb, Mo, and V were respectively set to% Ti,% Nb,%
When Mo and% V, α ≦ 50,000 × ((% Ti)
/ 48 + (% Nb) / 93 + (% Mo) / 96 + (%
V) / 51). A low-yield-ratio high-strength cold-rolled steel sheet excellent in hole expandability, characterized by being V) / 51).

(6) The above (1), (2) or (3)
Wherein the total volume fraction of martensite and retained austenite in the metal structure is 6% or more, and the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti,
The addition amounts of Nb, Mo, and V were respectively set to% Ti,% Nb,%
When Mo and% V, α ≦ 50,000 × ((% Ti)
/ 48 + (% Nb) / 93 + (% Mo) / 96 + (%
V) / 51) A low yield ratio high-strength plated steel sheet excellent in hole expandability, characterized in that:

(7) The above (1), (2) or (3)
% Slab to a slab having a composition consisting of the chemical components described in
When the content is determined, the finishing temperature is (900 + 50 ×% S
i) A hot-rolled sheet subjected to hot rolling at 50 ° C. or less and cold-rolled at 50 to 85% is subjected to ferrite at 700 to 900 ° C.
Austenite is annealed in a two-phase coexisting temperature range for 10 seconds to 5 minutes, and the average cooling rate between 700 ° C and 500 ° C is 1
It is characterized by cooling to 250 to 500 ° C. at a temperature of 120 to 120 ° C./sec, reheating if necessary, maintaining the temperature in the range of 250 to 600 ° C. for 30 seconds to 10 minutes, and then cooling to room temperature. The volume fraction of martensite and retained austenite in the metal structure is 6% or more in total, and the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb, Mo, V
Α = 50,000 × ((% Ti) / 48 + (% N
b) / 93 + (% Mo) / 96 + (% V) / 51), and a method for producing a low-yield-ratio high-strength cold-rolled steel sheet excellent in hole expandability.

(8) The above (1), (2) or (3)
% Slab to a slab having a composition consisting of the chemical components described in
When the content is determined, the finishing temperature is (900 + 50 ×% S
i) A hot-rolled sheet subjected to hot rolling at 50 ° C. or less and cold-rolled at 50 to 85% is subjected to ferrite at 700 to 900 ° C.
Austenite is annealed in a two-phase coexisting temperature range for 10 seconds to 5 minutes, and the average cooling rate between 700 ° C and 500 ° C is 1
It is characterized by cooling to 250 to 500 ° C. at a temperature of 120 to 120 ° C./sec, reheating if necessary, maintaining the temperature in the range of 250 to 600 ° C. for 30 seconds to 10 minutes, and then cooling to room temperature. The volume fraction of martensite and retained austenite in the metal structure is 6% or more in total, and the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb, Mo, V
Α = 50,000 × ((% Ti) / 48 + (% N
b) / 93 + (% Mo) / 96 + (% V) / 51), which is a method for producing a low-yield-ratio high-strength plated steel sheet having excellent hole expandability.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

First, C, Si, Mn, P, S, Al, T
The reasons for limiting the numerical values of i, Nb, Mo, V, and N will be described.

C is an essential element for realizing the structural strengthening by martensite and retained austenite at low cost. If C is less than 0.04%, the volume ratio of martensite and retained austenite decreases, so C is necessary. It is difficult to secure sufficient tensile strength. On the other hand, if C exceeds 0.14%, the volume ratio of martensite or retained austenite increases, and it is easy to increase the tensile strength, but sharp voids are formed at the shear end face, and the hole expandability is significantly deteriorated. .

It is well known that the addition of Si increases the strength of the steel sheet without significantly impairing the elongation of the steel sheet. In addition, the addition of Si significantly delays the progress of the pearlite and bainite transformations and inhibits the formation of coarse carbides, thereby increasing the hole expansion property. At the same time, the presence of martensite and residual austenite in the metal structure after cooling to room temperature facilitates the elongation in the tensile test.In addition, in the present invention, the metal after annealing by hot rolling under certain conditions is particularly used. The structure is mainly composed of ferrite near martensite,
It is strengthened by fine precipitates containing elements such as Nb, Mo, and V, and is useful in reducing the difference in deformation stress between adjacent structures. The low yield ratio characteristic of the present invention provides elongation and hole expansion in a tensile test. It is important to achieve gender balance. It significantly slows down the progress of the pearlite and bainite transformations and inhibits the formation of coarse carbides, improves hole-expandability, and facilitates the presence of martensite and residual austenite in the metal structure after cooling to room temperature, In order to increase the elongation, it is necessary to add more than 0.4%. However, when the addition amount exceeds 2.2%, hot rolling is performed so that ferrite near martensite is mainly contained in the metal structure after annealing and fine precipitates containing elements such as Ti, Nb, Mo, and V are strengthened. This makes it difficult to achieve a low yield ratio and hole expandability at the same time.

Mn is known as an element that lowers the free energy of austenite together with C and increases the hardenability of steel. To improve elongation at a required tensile strength, martensite and retained austenite are combined in a volume ratio of Mn. 1.2% for the purpose of having a metal structure existing at 6% or more
Add above. However, if the addition amount is excessive, the addition amount of Ti that does not deteriorate the hole expandability at a low yield ratio cannot be found, so the upper limit is 2.2%.

P is generally contained in steel as an unavoidable impurity, but if its amount exceeds 0.02%, cold rolling as well as toughness of a high-strength steel sheet having a tensile strength exceeding 590 MPa as in the present invention is performed. The properties are remarkably deteriorated, and it becomes difficult to mass-produce industrially.

S is generally contained in steel as an inevitable impurity, but if its content exceeds 0.01%, the adverse effect on the spot weldability of the steel sheet becomes significant.

Al is used as a deoxidizing element of steel, and is used in order to improve the material quality by suppressing the grain size of the hot-rolled material by AlN and suppressing the coarsening of the crystal grains in a series of heat treatment steps.
It is necessary to add 02% or more, but if it exceeds 0.5%, not only the cost is increased but also the surface properties are deteriorated.

Ti is added for the purpose of finely precipitating mainly ferrite in the vicinity of martensite to reduce the difference in deformation stress between adjacent structures, and to prevent S from becoming an expanded inclusion as MnS and deteriorating hole expandability. I do. In general, when Ti precipitates finely, the recrystallization temperature increases at the same time as the tensile strength increases, and the ferrite may be in an unrecrystallized state even in the two-phase coexistence temperature region of ferrite and austenite. By adjusting the hot rolling conditions after the addition of manganese, a metal structure containing martensite and retained austenite in a total volume ratio of at least 6% mainly composed of sufficiently recrystallized ferrite can be formed regardless of the annealing temperature. If the addition amount of Ti is less than 0.005%, there is no effect on the precipitation strengthening of ferrite in the vicinity of martensite, and when% S and% Ti are respectively S and Ti contents,
If Ti) / (% S) is less than 5, it is difficult to suppress the formation of expanded MnS, and the hole expandability cannot be improved.
However, when the amount of Ti exceeds 0.1%, not only the yield ratio increases due to significant precipitation strengthening of ferrite, but even if the hot rolling conditions are adjusted together with the Si content of the steel as specified in the present invention. Since it is difficult to recrystallize ferrite by short-time annealing, elongation in a tensile test is also small.

N is contained in steel as an unavoidable impurity, and precipitates Ti, Nb, Mo, V and nitrides to increase the strength of the steel. When the amount exceeds 0.006%, the tensile test is performed. , The elongation is deteriorated.

Nb, Mo, and V are also precipitation strengthening elements, like Ti, and finely precipitate mainly ferrite near martensite, thereby improving the hole expandability by reducing the deformation stress difference between adjacent structures. Then, a total of 0.005 to 0.1% may be added for the purpose of securing the required tensile strength. However, if the added amount exceeds 0.1%, Ti
At the same time, recrystallization of ferrite is significantly inhibited, and workability is deteriorated.

The steels containing these as main components include Cu, Sn, Z
Even if n, Zr, W, Cr, Ni, and B are contained in a total amount of 1% or less, the effect of the present invention is not impaired, and depending on the amount, the corrosion resistance may be improved.

Next, the reasons for limiting the manufacturing conditions will be described.
Its purpose is to expand the hole while maintaining a low yield ratio in a composite structure high-strength steel sheet that has a metal structure in which martensite, retained austenite and bainite are finely dispersed in a ferrite matrix and has improved elongation in tensile tests. In order to improve the ferrite, the ferrite itself is strengthened with fine precipitates containing elements such as fine Ti, Nb, Mo, and V mainly in the vicinity of martensite so that sharp voids are not formed at the phase boundary between ferrite and martensite at the shear end face. Is to do. High strength, low yield ratio and large elongation in the tensile test are characterized by the strengthening of the composite structure when the volume fraction of martensite and retained austenite is 6% or more. In order to clearly recognize the effect of plasticity, it is preferable that the volume fraction of retained austenite exceeds the volume fraction of martensite. Although the coexistence of bainite is not preferable in terms of exhibiting the characteristics of composite structure strengthening such as high strength, low yield ratio, and high elongation in a tensile test, the volume ratio of martensite and retained austenite is low. When it does not exceed 3 times, the tensile strength of the steel sheet can be increased with a smaller C content without significantly increasing the yield ratio, which is useful in the present invention. Further, the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α
%, Ti, Nb, Mo, and V were added to% Ti,
% ≦ Nb,% Mo,% V, α ≦ 50,000 ×
((% Ti) / 48 + (% Nb) / 93 + (% Mo) /
96 + (% V) / 51), the ferrite itself is finely divided into Ti, Nb, Mo, V, mainly in the vicinity of martensite.
And the formation of sharp voids at the phase boundary between ferrite and martensite at the sheared end face is suppressed, and hole expandability is improved. α /
((% Ti) / 48 + (% Nb) / 93 + (% Mo) /
When 96 + (% V) / 51) exceeds 50,000, precipitation strengthening of ferrite becomes insufficient, so that a sharp void is easily formed at a phase boundary between ferrite and martensite at a shear end face, and hole expandability is increased. to degrade.

The slab to be subjected to hot rolling is not particularly limited. That is, it may be any one manufactured with a continuous cast slab or a thin slab caster. It is also suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

In hot rolling, when% Si is the Si content, the finishing temperature must be (900 + 50 ×% Si) ° C. or less. This is because precipitation of carbonitrides of Ti, Nb, Mo, and V is performed so as not to abnormally raise the recrystallization temperature after the subsequent cold rolling, and when annealing is performed in a two-phase coexisting temperature range of ferrite and austenite. Regardless of the temperature, the volume fraction of martensite and retained austenite is 6 based on fully recrystallized ferrite.
%, And the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti,
The addition amounts of Nb, Mo, and V were respectively set to% Ti,% Nb,%
When Mo and% V, α ≦ 50,000 × ((% Ti)
/ 48 + (% Nb) / 93 + (% Mo) / 96 + (%
V) / 51) is strengthened mainly by ferrite near martensite with fine precipitates containing elements such as Ti, Nb, Mo and V to form a metal structure in which the difference in deformation stress between adjacent structures is reduced. The purpose is to let them. If the finishing temperature exceeds (900 + 50 ×% Si) ° C., the carbonitrides become finely dispersed and the tensile strength increases, but when annealing is performed in the two-phase coexisting temperature range of ferrite and austenite. Also, since unrecrystallized ferrite remains, elongation in a tensile test deteriorates. The cooling conditions and coiling temperature after hot rolling are not particularly limited.However, the coiling temperature is set to avoid large variations in material at both ends of the coil and to avoid deterioration in pickling properties due to an increase in scale thickness. Is 7
The temperature is set to 50 ° C. or less. In order to prevent the precipitation of Ti, Nb, Mo, and V from becoming insufficient and causing a variation in the material, 5 ° C.
Desirably, the temperature is not lower than 00 ° C.

The cold rolling may be performed under ordinary conditions, and the rolling ratio is set to 50% or more for the purpose of refining the metal structure after a series of heat treatments and maximizing the improvement of elongation in a tensile test. On the other hand, performing cold rolling at a rolling ratio exceeding 85% is not practical because a large cold rolling load is required.

First, the cold-rolled steel sheet is 700 to 900 ° C.
Ferrite and austenite in the dual temperature range of 10
Anneal for seconds to 5 minutes. This annealing forms a fine recrystallized structure in which ferrite and austenite coexist, and
An object of the present invention is to concentrate an austenite stabilizing element such as Mn or Mn in austenite to some extent, and to stabilize austenite at the time of a change in structure following a series of heat treatments. If the annealing temperature is lower than 700 ° C., recrystallization is insufficient and elongation in a tensile test is deteriorated. 900
Ti, Nb, Mo, V when annealed at a temperature exceeding
Is partially dissolved, the volume ratio of austenite becomes unnecessarily large, and the distribution ratio of C between austenite and ferrite is small, and the chemical stability of austenite deteriorates. Due to severe restrictions, a metal structure is intended, the necessary tensile strength is secured, and a low yield ratio makes it difficult to achieve both elongation and hole expandability in a tensile test. If the annealing time is less than 10 seconds, the carbide will not sufficiently form a solid solution, and even if the annealing temperature is high, only a small amount of austenite will be formed. Annealing longer than 5 minutes not only wastes energy, but also reduces productivity in a continuous line.

The annealed steel sheet is subsequently 250 to 500
C., at which time the cooling rate from 700 ° C. to 500 ° C. is set to 1 to 120 ° C. in order to avoid austenite formed in the two-phase coexisting temperature range from being transformed into pearlite.
/ Sec. When cooling is stopped at a temperature exceeding 500 ° C., the pearlite transformation starts rapidly, and austenite cannot remain, so that characteristics such as a low yield ratio and excellent hole expandability cannot be obtained. On the other hand, when the cooling end temperature is lower than 250 ° C., the majority of austenite is transformed into martensite, so that martensite is tempered by subsequent holding or reheating to precipitate cementite, and the yield ratio is high even if the strength is high. High, the elongation in the tensile test deteriorates, and the hole expandability is not good. If the cooling rate between 700 ° C. and 500 ° C. is less than 1 ° C./sec, pearlite is formed, the yield ratio is high, and the elongation and hole expandability in the tensile test are poor.
When the cooling is performed at a cooling rate exceeding 80 ° C./sec, the transformation from austenite to ferrite hardly occurs during the cooling, and particularly when the annealing temperature is high, the transformation is continued at 250 to 6 ° C.
The volume ratio of bainite generated by holding at 00 ° C. becomes large, the yield ratio is high even though the strength is high, and the elongation and hole expandability in the tensile test are poor.

Thereafter, in the present invention, after reheating as required, the temperature is kept in a temperature range of 250 to 600 ° C. for 30 seconds to 10 minutes, and then cooled to room temperature. In a tensile test derived from the strengthening of the composite structure with martensite and retained austenite by increasing the ferrite's ductility by precipitating C present in a supersaturated state in the ferrite at the time of cooling and increasing the ductility of the ferrite. The purpose is to maximize improvement in elongation and decrease in yield ratio. When the holding temperature exceeds 600 ° C., the solid solubility limit of C in ferrite is high, and not only does it have no effect, but pearlite and bainite are easily formed from austenite, the yield ratio is high, and the elongation and hole expandability in a tensile test are high. Is not good either. On the other hand, if the temperature is lower than 250 ° C., fine carbides are precipitated in the ferrite, and the ductility of the ferrite is reduced. Therefore, martensite and austenite are present in a total volume ratio of 6% or more. Is not good in elongation and hole spreadability.
If the holding time is less than 30 seconds, the precipitation of C is extremely insufficient, and the ductility of ferrite is not high, so that the workability is insufficient. On the other hand, when it is held for more than 10 minutes, the majority of austenite is transformed into bainite, the effect of strengthening the composite structure is not exhibited, and the workability is not good.

In this series of heat treatments, the holding temperature does not need to be constant as long as it is within the specified temperature range, and the fact that the cooling rate changes within the specified range during cooling does not impair the gist of the present invention. In particular, holding for 30 seconds to 10 minutes in a temperature range of 250 to 600 ° C. may be reheating after supercooling within this temperature range, and any of those illustrated in FIG. 1 is possible. Further, as long as the heat history is satisfied, the steel sheet may be heat-treated by any equipment such as a continuous annealing equipment or an in-line annealing continuous hot-dip galvanizing equipment. After heat treatment, temper rolling is performed to correct the shape, electro-galvanizing is performed to improve rust prevention, and surface treatment such as metal oxide film and organic film is performed to further improve rust prevention. Even if it is applied, even if the upper layer is further plated with iron to improve the press formability, the feature of the present invention of a high-strength steel sheet excellent in press formability including stretch flange processing is not impaired, It is preferable for achieving the object of the present invention because it leads to further improvement of rust resistance.

[0035]

Next, examples of the present invention will be described with reference to examples.

Example 1 A steel having the composition shown in Table 1 was heated to 1190 ° C., and at the finishing temperature shown in Table 2,
0-7.5mm hot-rolled steel strip, 540-670 ° C
Rolled up. After pickling, cold rolling is performed at a reduction ratio of 55 to 75% to obtain a cold-rolled steel strip having a thickness shown in Table 2, and then heat treatment is performed using a continuous annealing facility under the conditions shown in Table 2. Temper rolling at an elongation of 0.3 to 1.5% was performed to produce a cold-rolled steel sheet. The metal structure of the sample cut from this steel strip was observed with an optical microscope to determine the volume ratio of martensite and retained austenite, and the JIS No. 5 test piece was processed and subjected to a tensile test at room temperature to yield. The strength (YP), tensile strength (TS) and elongation (El) were determined. Further, a steel plate is cut out into a rectangle of 150 mm × 150 mm, and a hole having a diameter d ₀ = 10 mm is formed at the center thereof by about 12%.
After punching with a clearance of 5 mm, set the burr on the tester so that it is on the die side, and restrain with 6 tons of wrinkle holding force so that material does not flow into the expanded portion.
The hole is pushed and expanded with a 0 ° conical punch, and the hole diameter d when a crack penetrates the edge of the hole through the thickness is measured, and the rate of increase of the hole diameter, that is, λ = (dd− ₀ ) / d ₀ is expanded. Table 2 shows the results evaluated as percentages.

As is clear from this table, the sample of the present invention, Sample No. 2, 5, 7, 10, 12, 16, 22,
27, 32, and 34 to 36 have a tensile strength of 590 MPa or more, contain martensite and retained austenite in a total amount of 6% or more in their metal structure, have a small yield ratio of 0.7 or less, and have an elongation in a tensile test. Is large, the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, N
b, Mo, and V were added at% Ti,% Nb, and% M, respectively.
o,% V, α ≦ 50,000 × ((% Ti) /
48 + (% Nb) / 93 + (% Mo) / 96 + (% V)
/ 51), and the ferrite itself is strengthened by fine precipitates containing elements such as Ti, Nb, Mo and V mainly in the vicinity of martensite, and the difference in deformation stress between adjacent structures is reduced, so that the hole expandability is also improved. Excellent. On the other hand, even in the case of the component steel of the present invention, when the conditions of the heat treatment performed after the hot rolling or the cold rolling are inappropriate, the sample No. 4, 8, 9, 11,
13-15, 17-21, 28-31, the volume ratio of the combined martensite and retained austenite is 6
%, The volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, T
i, Nb, Mo, and V were added in the amounts of% Ti,% N, respectively.
α is 50,000 × ((%
Ti) / 48 + (% Nb) / 93 + (% Mo) / 96 +
(% V) / 51), the press workability is improved because any one of the following is not satisfied: high yield strength, low yield ratio, high elongation in tensile test, and excellent hole expandability. Or low elongation at a low yield ratio, large elongation in a tensile test, excellent hole expandability, and low strength even with good press workability. On the other hand, in steels other than the components of the present invention, sample Nos. 1, 3, 6, 23, 26, the combined volume fraction of martensite and retained austenite is 6% or more, or the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α.
%, Ti, Nb, Mo, and V were added to% Ti,
% ≦ Nb,% Mo,% V, α ≦ 50,000 ×
((% Ti) / 48 + (% Nb) / 93 + (% Mo) /
96 + (% V) / 51), and the sample No. 23, 26, 37, and 38, the content of martensite and retained austenite is 6% or more in total; the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%; When the amounts are% Ti,% Nb,% Mo and% V, respectively, α ≦ 50,000 × ((% Ti) / 48 + (% Nb) /
93 + (% Mo) / 96 + (% V) / 51), either of high strength, low yield ratio, high elongation in tensile test, or excellent hole expandability. The press workability is not improved because it is not satisfied, or the elongation in a tensile test is large at a low yield ratio, the hole expandability is excellent, and the strength is low even if the press workability is good.

[0038]

[Table 1]

[0039]

[Table 2]

Example 2 A steel having the composition shown in Table 1 was heated to 1220 ° C., and at the finishing temperature shown in Table 3,
0-6.0mm hot rolled steel strip, 580-680 ° C
Rolled up. After the pickling, cold rolling was performed at a reduction ratio of 55 to 73% to obtain a cold-rolled steel strip having a thickness shown in Table 3, and then using a continuous hot-dip galvanizing equipment of an in-line annealing method.
Then, heat treatment was performed under the following conditions and temper rolling at an elongation of 0.8 to 1.2% was performed to produce a hot-dip galvanized steel sheet (GI) and an alloyed hot-dip galvanized steel sheet (GA). The metal structure of the sample cut from this steel strip was observed with an optical microscope,
By obtaining the volume ratio of the combined martensite and retained austenite, and processing the JIS No. 5 test piece and performing a tensile test at room temperature, the yield strength (YP),
Tensile strength (TS) and elongation (El) were determined. Also 150
A steel plate is cut out into a rectangle of mm × 150 mm, a hole having a diameter of d ₀ = 10 mm is punched out at the center thereof with a clearance of about 12%, and then the burr is set on a testing machine so as to be on the die side. After restricting the material so that it does not flow into the expanded portion by force, expand the hole with a 60 ° conical punch, measure the hole diameter d when the crack penetrates the edge of the hole, and increase the hole diameter. That is, λ = (dd−d
Table 3 shows the results of the evaluation of ₀ ) / d ₀ as the hole expansion ratio.

As is clear from this table, the sample of the present invention, Sample No. 39, 40, 43 to 45 are 590 MPa
While having the above tensile strength, its metal structure contains 6% or more of martensite and retained austenite in total, the yield ratio is as small as 0.7 or less, the elongation in the tensile test is large, and the martensite is large. , The volume fraction of the hard phase structure of retained austenite and bainite is α
%, Ti, Nb, Mo, and V were added to% Ti,
Α ≦ 50,000 × when% Nb,% Mo,% V
((% Ti) / 48 + (% Nb) / 93 + (% Mo) /
96 + (% V) / 51), and the ferrite itself is strengthened by fine precipitates containing elements such as Ti, Nb, Mo and V mainly in the vicinity of martensite, and the difference in deformation stress between adjacent structures is reduced. Also, the hole expandability is excellent. On the other hand, even in the case of the component steel of the present invention, when the conditions of the heat treatment performed after the hot rolling or the cold rolling are inappropriate, the sample No. 46
The combined volume ratio of martensite and retained austenite is less than 6%, or martensite,
When the volume fraction of the hard phase structure of retained austenite and bainite is α%, and the added amounts of Ti, Nb, Mo and V are% Ti,% Nb,% Mo and% V, α is 500
00 × ((% Ti) / 48 + (% Nb) / 93 + (% M
o) / 96 + (% V) / 51), and although high strength, any one of low yield ratio, high elongation in tensile test, and excellent hole expandability is not satisfied. No improvement in workability, or a low yield ratio, large elongation in a tensile test, excellent hole expandability, and low strength even with good press workability. On the other hand, in steels other than the components of the present invention, sample Nos. As in 41 and 42, the volume ratio of the combined martensite and retained austenite is 6% or more, or the volume ratio of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb, Mo, V % T
When i,% Nb,% Mo, and% V, α ≦ 50,000
× ((% Ti) / 48 + (% Nb) / 93 + (% Mo)
/ 96 + (% V) / 51), and the sample No. 47 and 48, the total content of martensite and retained austenite is 6% or more, the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, and the added amount of Ti, Nb, Mo and V is%. Α ≦ 50 when Ti,% Nb,% Mo,% V
000 × ((% Ti) / 48 + (% Nb) / 93 + (%
Mo) / 96 + (% V) / 51), either of high strength, low yield ratio, high elongation in tensile test, or excellent hole expandability are not satisfied. The press workability is not improved, or the elongation in a tensile test is large at a low yield ratio, the hole expandability is excellent, and the strength is low even if the press workability is good.

[0042]

[Table 3]

[0043]

As described in detail above, according to the present invention, in a low-yield-ratio high-strength steel sheet in which martensite, retained austenite and bainite are finely dispersed in a ferrite matrix, not only elongation in a tensile test but also hole expansion is obtained. The performance is also improved. That is, the tensile strength is 590 to 880.
It is possible to apply high-strength steel sheets of MPa to parts that are difficult to stretch and flange by press working, and to reduce the weight while improving the functions that each should have in the fields of automobiles, home appliances, construction, etc. Has an extremely great effect on industry.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating some heat treatment conditions in the present invention.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Eishin Murasa 1 Kimitsu, Kimitsu City Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Jun Itami 1 Kimitsu, Kimitsu City Nippon Steel Corporation F-term in Tsu Works (reference) 4K037 EA01 EA02 EA05 EA06 EA15 EA16 EA17 EA18 EA19 EA23 EA25 EA27 EA28 EA31 EA32 EB05 EB09 EB12 FA02 FC04 FC05 FH01 FJ05 FJ06 FK02 FK03 FK08 FL01 FL05

Claims (8)

[Claims] 1. A mass% of C: 0.04 to 0.14%,
Si: 0.4 to 2.2%, Mn: 1.2 to 2.4%,
P: 0.02% or less, S: 0.01% or less, Al: 0.
002-0.5%, Ti: 0.005-0.1%, N:
0.006% or less, and when% S and% Ti are S and Ti content, respectively, (% Ti) / (% S) ≧
5. A low-yield-ratio high-strength cold-rolled steel sheet which satisfies No. 5 and comprises the balance of Fe and inevitable impurities and has excellent hole expandability. 2. C: 0.04 to 0.14% by mass%,
Si: 0.4 to 2.2%, Mn: 1.2 to 2.4%,
P: 0.02% or less, S: 0.01% or less, Al: 0.
002-0.5%, Ti: 0.005-0.1%, N:
0.006% or less, and further contains at least one of Nb, Mo, and V in an amount of 0.005 to 0.1%, and further contains% S and% T.
When i is S and Ti content respectively, (% Ti) /
A low-yield-ratio high-strength cold-rolled steel sheet which satisfies (% S) ≧ 5 and is composed of a balance of Fe and inevitable impurities, and is excellent in hole expandability. 3. B: 0.0010 to 0.00% by mass
The low-yield-ratio high-strength cold-rolled steel sheet according to claim 1 or 2, which contains 3%. 4. The low-yield-ratio high-strength plated steel sheet according to claim 1, 2 or 3, which is excellent in hole expandability. 5. The chemical composition according to claim 1, 2 or 3, wherein the volume fraction of martensite and retained austenite in the metal structure is 6% in total.
As described above, the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, N
b, Mo, and V were added at% Ti,% Nb, and% M, respectively.
o,% V, α ≦ 50,000 × ((% Ti) /
48 + (% Nb) / 93 + (% Mo) / 96 + (% V)
/ 51) A low-yield-ratio high-strength cold-rolled steel sheet having excellent hole expandability, characterized in that: 6. The chemical composition according to claim 1, 2 or 3, wherein the volume fraction of martensite and retained austenite in the metal structure is 6% in total.
As described above, the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, N
b, Mo, and V were added at% Ti,% Nb, and% M, respectively.
o,% V, α ≦ 50,000 × ((% Ti) /
48 + (% Nb) / 93 + (% Mo) / 96 + (% V)
/ 51) A low yield ratio and high strength plated steel sheet having excellent hole expandability, characterized in that: 7. When the slab having the chemical composition according to claim 1, 2, or 3 has% Si as the Si content, the finishing temperature is (900 + 50 ×% Si).
7 ° C. or lower, and a cold-rolled sheet subjected to 50-85% cold rolling is annealed at 700-900 ° C. in a two-phase coexisting temperature range of ferrite and austenite for 10 seconds to 5 minutes.
The average cooling rate between 00 ° C and 500 ° C is 1-12.
After cooling to 250 to 500 ° C. at 0 ° C./sec and reheating if necessary,
A metal alloy having a total volume fraction of martensite and retained austenite of 6% or more, and a hard phase structure of martensite, retained austenite and bainite, characterized by being cooled to room temperature after holding for 10 to 10 minutes. Α ≦ 50000 × ((% Ti) / 48 + (% Nb), when the volume ratio of α is% and the added amounts of Ti, Nb, Mo and V are% Ti,% Nb,% Mo and% V, respectively.
/ 93 + (% Mo) / 96 + (% V) / 51), which is a method for producing a low-yield-ratio high-strength cold-rolled steel sheet having excellent hole expandability. 8. When the slab having the chemical composition according to claim 1, 2, or 3 has% Si as the Si content, the finishing temperature is (900 + 50 ×% Si).
7 ° C. or lower, and a cold-rolled sheet subjected to 50-85% cold rolling is annealed at 700-900 ° C. in a two-phase coexisting temperature range of ferrite and austenite for 10 seconds to 5 minutes.
The average cooling rate between 00 ° C and 500 ° C is 1-12.
After cooling to 250 to 500 ° C. at 0 ° C./sec and reheating if necessary,
A metal alloy having a total volume fraction of martensite and retained austenite of 6% or more, and a hard phase structure of martensite, retained austenite and bainite, characterized by being cooled to room temperature after holding for 10 to 10 minutes. Α ≦ 50000 × ((% Ti) / 48 + (% Nb), when the volume ratio of α is% and the added amounts of Ti, Nb, Mo and V are% Ti,% Nb,% Mo and% V, respectively.
/ 93 + (% Mo) / 96 + (% V) / 51), which is a method for producing a low-yield-ratio high-strength plated steel sheet having excellent hole expandability.