JP5483562B2 - High-strength cold-rolled steel sheet with an excellent balance between elongation and stretch flangeability - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in workability used for automobile parts and the like, and more particularly, to a high-strength cold-rolled steel sheet having an improved balance between elongation (total elongation) and stretch flangeability.

  For example, steel sheets used for automobile frame parts and the like are required to have high strength for the purpose of collision safety and fuel efficiency reduction by reducing the weight of the car body, and excellent forming process for processing into complex frame parts Sex is also required.

  For this reason, provision of a high-strength steel sheet having a tensile strength (TS) of 980 MPa or higher and an improved balance between elongation (total elongation; El) and stretch flangeability (hole expansion rate: λ) is eagerly desired. For example, the tensile strength TS is 980 MPa or more, TS × El is 17000 MPa ·% or more, and TS × El × λ is 1000000 MPa ·% ·% or more (more preferably, the tensile strength TS is 980 MPa or more, TS × El is 18000 MPa ·% or more, and TS × El × λ is 1300000 MPa ·% ·% or more).

  In response to the above needs, many high-strength steel sheets with improved balance between stretch and stretch flangeability have been proposed based on various structural control concepts, but the balance between stretch and stretch flangeability is at the above desired level. At present, there are only a few things that satisfy both requirements.

  For example, Patent Document 1 discloses a high-tensile cold-rolled steel sheet containing 1.6 to 2.5 mass% in total of at least one steel type of Mn, Cr, and Mo and substantially consisting of a single-phase structure of martensite. In the steel sheet with a tensile strength of 980 MPa, the hole expansion ratio (stretch flangeability) λ is 100% or more, but the elongation El does not reach 10%, and the above demand level is satisfied. None (see examples of the invention in Tables 2, 4, 6 and 8 of the same document).

  Patent Document 2 discloses a high-tensile steel plate having a two-phase structure of ferrite with an area ratio of 65 to 85% and the balance being tempered martensite.

  Patent Document 3 discloses a high-tensile steel sheet having a two-phase structure in which the average crystal grain sizes of ferrite and martensite are both 2 μm or less and the volume ratio of martensite is 20% or more and less than 60%.

  The high-strength steel sheets disclosed in Patent Documents 2 and 3 both have a high deformability and a large amount of ferrite to ensure elongation exceeding 10%, but satisfy the above desired level. (See the invention examples in Table 2 of Patent Document 2 and the examples in Tables 2 and 4 of Patent Document 3) And the invention which concerns on these high-tensile steel sheets controls the area ratio of a ferrite and a hard 2nd phase, and also controls the particle size of these both phases. However, the invention relating to these high-strength steel sheets does not contain V, Ti, or Nb, but contains V alone, or Ti and / or Nb in addition to V, and finely converts carbonitrides of these elements. The technical idea of the present invention is clearly different from that of the present invention, which is characterized by coexistence of an increase in strength due to precipitation and securing of stretch flangeability by preventing coarsening of the carbonitride and cementite.

  Patent Document 4 discloses that ferrite is an area ratio of 50 to 96%, and the balance is composed of a composite structure of martensite and tempered martensite, and contains one or more of V, Ti and Nb for precipitation strengthening. A tensile steel sheet is disclosed. The invention related to the high-strength steel sheet disclosed in Patent Document 4 is characterized by enhancing the strain age hardening characteristics of the steel sheet, and does not control the size distribution of precipitates of the above elements, and has a tensile strength of 980 MPa class. It has not been obtained (see the invention example in Table 3 of the same document). V or Ti alone and / or Nb in addition to V, clearly different in technical idea from the present invention, which is characterized by an increase in strength due to fine precipitation of carbonitrides of these elements It is.

  Patent Document 5 discloses a high-tensile cold-rolled steel sheet having a two-phase structure of ferrite and martensite having a crystal grain size of 2 μm or less and containing one or more of V, Ti and Nb for the purpose of strength adjustment. Has been. The invention related to the high-tensile cold-rolled steel sheet disclosed in Patent Document 5 is that the structure at the end of winding is martensite or bainite or a mixture thereof by rapidly cooling from the finishing temperature of hot rolling to 550 ° C or lower. It is in common with the present invention in that it is a low temperature transformation phase and suppresses the precipitation of carbides. However, the invention related to the high-tensile cold-rolled steel sheet disclosed in Patent Document 5 performs a long-time heat treatment for 10 hours or more at a temperature of 600 ° C. or higher and Ac1 point or lower between hot rolling and cold rolling. (Refer to paragraph [0040] of the same document), and by making the ferrite grains finer during annealing after cold rolling by precipitating carbides uniformly in such a temperature range, elongation and stretch flangeability can be achieved. It is characterized by securing. However, heat treatment for 10 hours or more is not preferable from the viewpoint of productivity, and in annealing after cold rolling, V alone or in addition to V contains one or two of Ti and Nb before recrystallization of the ferrite phase. The recrystallization ferrite grains are refined by finely precipitating the precipitates to be produced in a relatively short time, and the technical idea is clearly different from the present invention characterized by ensuring strength, elongation and stretch flangeability. .

JP 2002-161336 A JP 2004-256872 A JP 2004-232022 A JP 2004-52071 A JP 2005-213603 A

  Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet with improved formability and a method for producing the same, with an improved balance between elongation and stretch flangeability.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Si: 3.0% or less (including 0%),
Mn: 0.1 to 5.0%,
P: 0.1% or less (including 0%),
S: 0.010% or less (including 0%),
Al: 0.001 to 0.10%,
V: 0.10 to 0.50%
In order to satisfy the requirements, and the remainder has a component composition consisting of iron and inevitable impurities,
In addition to containing ferrite, which is a soft first phase, in an area ratio of 10 to 80%, the residual austenite, martensite, and the mixed structure of residual austenite and martensite are less than 5% (including 0%) in total area ratio There, the balance being a hard second phase, has a structure consisting of martensite and / or tempering bainite tempering,
The average particle diameter of the ferrite is 2 μm or less in terms of equivalent circle diameter,
The distribution state of precipitates present in the hard second phase in contact with the ferrite is
There are 3 or less cementite particles having an equivalent circle diameter of 0.1 μm or more per 1 μm ^{2 of the} hard second phase,
Of the particles containing V present in the form of precipitates in the ferrite, particles having an equivalent circle diameter of 8 nm or more and less than 20 nm account for 60% or more on a number basis with respect to the total number of particles containing V.
And, among the particles comprising the V present in the form of precipitates in the ferrite and the hard second phase, a circle equivalent diameter 20nm or more of the particles, are three or der less per 1 [mu] m ^2,
The tensile strength TS is 980 MPa or more, the product TS × El of the tensile strength TS and the elongation El is 17000 MPa ·% or more, and the product TS × El × λ of the tensile strength TS, the elongation El and the stretch flangeability λ is 1000000 MPa ·%. % Is a high-strength cold-rolled steel sheet excellent in the balance between elongation and stretch flangeability.

The invention described in claim 2
The component composition further includes Ti and / or Nb so as to simultaneously satisfy the following formulas 1 to 3,
Of the particles containing one or more of V, Ti and Nb present in the form of precipitates in the ferrite, particles having an equivalent circle diameter of 8 nm or more and less than 20 nm are 1 of V, Ti and Nb. Occupies 60% or more on a number basis with respect to the total number of particles containing two or more species,
Of the particles containing one or more of V, Ti and Nb present in the form of precipitates in the ferrite and the hard second phase, particles having an equivalent circle diameter of 20 nm or more are 3 per 1 μm ^2. Is less than
A high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1.
Formula 1: log ([% Ti] × [% C]) <− 1.6
Formula 2: log ([% Nb] × [% C]) <− 2.0
Formula 3: ([% V] / 51 + [% Ti] / 48 + [% Nb] / 93) × 51 ≦ 0.50%
Here, [% X] means the content (mass%) of the element X.

The invention according to claim 3
Ingredient composition further
Cr: 0.01 to 1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2.

The invention according to claim 4
Ingredient composition further
Mo: 0.02 to 1.0%,
Cu: 0.05 to 1.0%,
The high strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to any one of claims 1 to 3, wherein Ni is one or more of 0.05 to 1.0%. .

The invention described in claim 5
Ingredient composition further
Ca: 0.0005 to 0.01%, and / or
Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to any one of claims 1 to 4.

  According to the present invention, an appropriate amount of a hard second phase having a high deformability is mainly used in a dual phase structure steel composed of ferrite as a soft first phase and tempered martensite and / or tempered bainite as a hard second phase. In addition, by controlling the presence state of precipitates containing V alone or in addition to V existing in ferrite and hard second phase, or Ti, Nb in addition to V, while ensuring elongation, It has become possible to improve stretch flangeability, and to provide a high-strength cold-rolled steel sheet that has a better balance between stretch and stretch flangeability and has better formability.

  The present inventors mainly have a multiphase structure composed of ferrite as a soft first phase and tempered martensite and / or tempered bainite as hard second phase (hereinafter sometimes referred to as “tempered martensite”). Focusing on high-strength steel sheets with high strength, it is considered that if stretch flangeability can be improved while securing elongation, a high-strength steel sheet capable of satisfying the above-mentioned desired level can be obtained, which affects the balance between strength, elongation and stretch flangeability. We have conducted intensive studies such as investigating the effects of various factors. As a result, not only the ratio of ferrite, but also V alone present in ferrite and hard second phase, or carbon / nitride containing Ti and / or Nb in addition to V (hereinafter referred to as “MX type compound”) .)), It has been found that stretch flangeability can be improved while securing elongation, and the present invention has been completed based on this finding.

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on a multiphase structure similar to that of the steel sheets disclosed in Patent Documents 2 to 5, but is particularly MX type compound present in ferrite and hard second phase. This is different from the steel sheets disclosed in Patent Documents 2 to 5 described above in that the presence state of the steel sheet is controlled.

<Ferrite as soft first phase: 10 to 80% in area ratio>
In a multiphase steel such as ferrite-tempered martensite, deformation is mainly handled by ferrite having high deformability. Therefore, the elongation of the duplex steel such as ferrite-tempered martensite is mainly determined by the area ratio of ferrite.

  In order to ensure the target elongation, the area ratio of ferrite needs to be 10% or more (preferably 15% or more, more preferably 25% or more). However, since the strength cannot be secured when the ferrite is excessive, the area ratio of the ferrite is 80% or less (preferably 70% or less, more preferably 60% or less).

  In a multiphase structure steel such as ferrite-tempered martensite, the balance between strength and elongation depends not only on the area ratio of ferrite but also on the presence form of ferrite. That is, in a state where the ferrite particles are connected to each other, stress concentrates on the ferrite side having high deformability and only the ferrite bears deformation, so that it is difficult to obtain an appropriate balance between strength and elongation. On the other hand, when the ferrite particles are surrounded by tempered martensite particles and / or bainite particles, which are the hard second phase, the hard second phase is forcibly deformed, so that the hard second phase is also deformed. The balance between strength and elongation is improved.

The existence form of ferrite is evaluated by, for example, the number of points where a line segment having a total length of 1000 μm intersects a ferrite grain boundary (interface between ferrite particles) or a ferrite-hard second phase interface in a region having an area of 40000 μm ² or more. be able to. In order to effectively exhibit the above action, the preferable condition of the existence form of ferrite is (“number of intersections with ferrite-hard second phase interface”) / (“number of intersections with ferrite grain boundary” + “ferrite-hard” The number of intersections with the second phase interface ”) is 0.5 or less.

<Residual austenite, martensite, and mixed structure of retained austenite and martensite: less than 5% (including 0%) in total of area ratio, remaining: second phase tempered martensite and / or tempered bainite Organization>
In order to prevent embrittlement while securing strength, it is effective to make the region excluding ferrite mainly a structure tempered with martensite and / or bainite (structure composed of tempered martensite and / or tempered bainite). is there. At that time, if there is residual austenite or tempered martensite (hereinafter, “Martensite” simply means martensite that has not been tempered), stress concentrates around it, Since breakage is likely to occur, deterioration of stretch flangeability can be prevented by reducing residual austenite, martensite, and their mixed structure as much as possible.

  In order to effectively exhibit the above-described action, the retained austenite, martensite and mixed structure thereof are less than 5% (preferably 0%) in the total area ratio, and the balance is the second phase. The structure is composed of martensite and / or tempered bainite.

<Average diameter of ferrite: equivalent circle diameter of 2 μm or less>
By making the ferrite finer, the interface between the ferrite and the hard second phase increases and the stress is dispersed, so that stretch flangeability is improved.

<Three or less cementite particles having a circle equivalent diameter of 0.1 μm or more in the distribution state of precipitates present in the hard second phase in contact with the ferrite interface are 3 or less per 1 μm ^{2 of the} hard second phase>
The cementite precipitated in the hard second phase in contact with the ferrite can serve as a starting point for fracture. If the cementite particles are coarse, the stress concentration during deformation becomes excessive and stretch flangeability cannot be secured. Therefore, in order to secure stretch flangeability, it is necessary to control the size and density of the cementite particles.

In order to ensure stretch flangeability, the number of coarse cementite particles having an equivalent circle diameter of 0.1 μm or more is 3 or less, preferably 2.5 or less, more preferably 2 or less, per 1 μm ^{2 of the} hard second phase. Restrict.

<Of the particles containing V present in the form of precipitates in the ferrite, particles with an equivalent circle diameter of 8 nm or more and less than 20 nm are 60% or more on a number basis with respect to the total number of particles containing V>
Controlling the size of the precipitate containing V is the most important requirement for exerting the effects of the present invention. By finely dispersing precipitates containing V in ferrite which is a soft phase, ferrite can be strengthened by precipitation. The precipitation strengthening amount is expressed by the following formula 4. The precipitation strengthening amount increases as the volume fraction of the precipitated particles increases and as the size of the precipitated particles decreases. Strengthening the ferrite increases the strength of the entire duplex steel, reduces the strength difference between the ferrite and the hard second phase, and suppresses breakage at the ferrite-hard second phase interface. Flangeability is improved. However, when the precipitate size falls below a certain critical value, the precipitate is sheared by dislocation, so the precipitation strengthening amount is saturated, and the dislocations easily move on the specific slip surface where the precipitation particles are sheared, A reduction in ductility occurs due to non-uniform deformation.

  In order to effectively exert the above action, among the particles containing V present in the form of precipitates in ferrite, the equivalent circle diameter is 8 nm or more and less than 20 nm, preferably 8 nm or more and less than 17 nm, more preferably 8 nm or more and less than 15 nm. The precipitate size is controlled so that particles account for 60% or more on a number basis.

<Of the particles containing V present in the form of precipitates in the ferrite and the hard second phase, 3 or less particles having an equivalent circle diameter of 20 nm or more per 1 μm ² >
Precipitates containing V, such as VC, have extremely high rigidity and critical shear stress compared to the parent phase, so even if the periphery of the precipitate is deformed, the precipitate itself is not easily deformed. A large strain is generated at the interface between the matrix phase and the precipitate, and fracture occurs. For this reason, if there are a large amount of coarse precipitates containing V of 20 nm or more, stretch flangeability deteriorates. Therefore, stretch flangeability can be improved by restricting the density of coarse precipitates containing V.

In order to effectively exert the above-described action, among the particles containing V present in the form of precipitates in ferrite and the hard second phase, 3 or less particles having an equivalent circle diameter of 20 nm or more per 1 μm ² , preferably Is limited to 2 or less, more preferably 1 or less.

  Hereinafter, a method for measuring the area ratio of each phase, the average particle diameter of ferrite, the size and density of precipitates, and the existence form of ferrite will be described.

[Measurement of area ratio of each phase]
First, regarding the area ratio of each phase, each test steel sheet was mirror-polished, corroded with a 3% nital solution to reveal the metal structure, and then a scanning type with a magnification of 2000 times for approximately 5 fields of 40 μm × 30 μm area. An electron microscope (SEM) image was observed and the area of the ferrite was determined by measuring 100 points per field of view by a point calculation method. In addition, a region containing cementite was determined as a hard second phase by image analysis, and the remaining region was retained austenite, martensite, and a mixed structure of retained austenite and martensite. The area ratio of each phase was calculated from the area ratio of each region.

[Measurement method of average particle diameter of ferrite]
The equivalent circle diameter was calculated from the area of each ferrite grain measured in the measurement of the area ratio.

[Measurement method of precipitate size and density]
Regarding the size of cementite and the density of the cementite, an extraction replica sample of each test steel plate was prepared, and a transmission electron microscope (TEM) image at a magnification of 50000 times was observed for three fields of view of 2.4 μm × 1.6 μm.

And about a hard 2nd phase which contact | connects an interface with a ferrite, a black part is discriminate | determined from cementite particle | grains from the contrast of an image, and it marks with an image analysis software from the area A of each of said marked cementite particle | grains D ( D = 2 × (A / π) ^1/2 ), and the number of cementite particles having a predetermined size per unit area was determined.

  For the size of particles containing Ti and / or Nb in addition to V alone or in addition to V and the abundance thereof, an extraction replica sample of each test steel plate was prepared, and a magnification of 100,000 times for a region 3 fields of about 900 nm × 770 nm. A transmission electron microscope (TEM) image was observed. The size of the MX type compound was measured in the same manner as the cementite measurement method. The size was measured by confirming the presence of V, Ti and Nb in the precipitate using EDX or EELS attached to FE-TEM.

[Method for measuring the presence of ferrite]
Each test steel plate was polished to a mirror surface and corroded with 3% nital solution to reveal the metal structure. Then, 20 lines of 50 μm were drawn in 10 fields of 80 μm × 60 μm region, The number N _α of ferrite grain boundaries intersecting with and the number N _α-TM of the ferrite-hard second phase interface are measured. And the ratio N ( _alpha) / (N ( _alpha ) + N ( _alpha ) _-TM ) of the ferrite grain boundary which occupies for a grain boundary and an interface is calculated | required as an evaluation index of a ferrite existing form. The small value of N _α / (N _α + N _α-TM ) means that there are few regions where the ferrite particles and ferrite particles are continuous, that is, the ferrite particles are not continuous and are surrounded by the hard second phase. It shows that.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.05-0.30%
C is an important element that affects the area ratio of the hard second phase and the amount of cementite precipitated in the hard second phase, and affects the strength, elongation, and stretch flangeability. If it is less than 0.05%, the strength cannot be secured. On the other hand, if it exceeds 0.30%, not only a large amount of strain enters during quenching, but also the amount of cementite increases and it becomes difficult to recover the dislocations, so the elongation and stretch flangeability deteriorate. Also, from the viewpoint of securing weldability, it is desirable that the C content is low, so 0.30% is made the upper limit.

  The range of C content is preferably 0.10 to 0.25%, more preferably 0.14 to 0.20%.

Si: 3.0% or less (including 0%)
Si has an effect of suppressing the coarsening of cementite particles during tempering, and is a useful element that contributes to both elongation and stretch flangeability. Moreover, since it contributes to strengthening of steel by solid solution strengthening, it is a useful element for obtaining the required strength. If it exceeds 3.0%, the ferrite becomes brittle and TS × El decreases. The range of Si content becomes like this. Preferably it is 0.50 to 2.5%, More preferably, it is 1.0 to 2.2%.

Mn: 0.1 to 5.0%
Mn, like Si, has the effect of suppressing cementite coarsening during tempering, contributes to solid solution strengthening, and enhances the deformability of the hard second phase to increase elongation and stretch flangeability. Contributes to the coexistence of Moreover, there exists an effect which expands the range of the manufacturing conditions from which a hard 2nd phase is obtained by improving hardenability. If the content is less than 0.1%, the above effects cannot be sufficiently exhibited, so that it is impossible to achieve both elongation and stretch flangeability. On the other hand, if it exceeds 5.0%, the reverse transformation temperature becomes too low and recrystallization becomes impossible. And the balance of growth cannot be secured. The range of Mn content is preferably 0.5 to 2.5%, more preferably 1.2 to 2.2%.

P: 0.1% or less P is unavoidably present as an impurity element, and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and embrittles the grain boundaries to increase stretch flangeability. Since it deteriorates, it is made 0.1% or less. Preferably it is 0.05%, More preferably, it is 0.03% or less.

S: 0.005% or less S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of a crack when expanding holes, thereby reducing stretch flangeability. . More preferably, it is 0.003% or less.

N: 0.01% or less N is also unavoidably present as an impurity, and lowers the elongation and stretch flangeability by strain aging.

Al: 0.001 to 0.10%
Al is added as a deoxidizing element and has the effect of making inclusions finer. Moreover, it combines with N to form AlN and reduces the solid solution N that contributes to the occurrence of strain aging, thereby preventing elongation and stretch flangeability from being deteriorated. If it is less than 0.001%, solid solution N remains in the steel, so strain aging occurs and elongation and stretch flangeability cannot be secured. On the other hand, if it exceeds 0.1%, austenite formation during heating is inhibited. The area ratio of the hard second phase cannot be secured, and the stretch flangeability cannot be secured.

V: 0.10 to 0.50%
V forms fine MX type compounds (generic name for carbides, nitrides, and carbonitrides), and these fine MX type compounds act as particles that pin the growth of austenite during heating during annealing. Thus, it contributes to the refinement of ferrite grains and refines the structure after hot rolling to enhance stretch flangeability. When the V content exceeds the above upper limit value, a coarse MX type compound is likely to be precipitated, and as described above, it becomes a starting point of breakage at the time of hole expansion, so that stretch flangeability is deteriorated. In addition, since these elements have an action of strongly suppressing recrystallization, when the addition amount exceeds the above upper limit value, the structure processed during hot rolling cannot be recrystallized during annealing, and the strength and elongation are increased. Balance cannot be secured. On the other hand, if the V content is less than the lower limit, the ferrite grain refinement effect cannot be sufficiently obtained, and the precipitation strengthening effect described above cannot be sufficiently obtained, so that a balance between strength and elongation can be secured. Disappear.

  The steel of the present invention basically contains the above components, and the balance is substantially iron and impurities. In addition, the following allowable components can be added as long as the effects of the present invention are not impaired.

Containing Ti and / or Nb so that the following re-expressions 1 to 3 are satisfied simultaneously Re-representation 1: log ([% Ti] × [% C]) <− 1.6
Reprinting formula 2: log ([% Nb] × [% C]) <− 2.0
Reprinting formula 3: ([% V] / 51 + [% Ti] / 48 + [% Nb] / 93) × 51 ≦ 0.50%
Here, [% X] means the content (mass%) of the element X.
Ti and Nb, like V, form a fine MX-type compound, and this fine MX-type compound acts as a particle to pin the growth of austenite during heating during annealing, thereby refining ferrite grains. Stretch flangeability is improved by miniaturizing the structure after hot rolling. If the solubility product of Ti and Nb and carbon is not smaller than the equilibrium solubility product when heated at 1200 ° C. (see “Hot rolling conditions” in “Preferred production method of steel sheet of the present invention” described later) (that is, the above formula 1 and If not satisfying 2), undissolved Ti and Nb remain as coarse carbides at the time of heating the slab before hot rolling, and as described above, it becomes the starting point of fracture when expanding the hole, and the stretch flangeability is deteriorated. . On the other hand, if the total content in terms of V due to the addition of Ti and Nb exceeds the upper limit of the above formula 3, a coarse MX-type compound is likely to precipitate, and the stretch flangeability is deteriorated. In addition, since these elements have an action of strongly suppressing recrystallization, when the addition amount exceeds the above upper limit value, the structure processed during hot rolling cannot be recrystallized during annealing, and the strength and elongation are increased. Balance cannot be secured.

Cr: 0.01 to 1.0%
Cr is a useful element that can improve stretch flangeability by suppressing the growth of cementite. If the addition is less than 0.01%, the above-described effects cannot be exhibited effectively. On the other hand, if the addition exceeds 1.0%, coarse Cr ₇ C ₃ is formed, and the stretch flangeability deteriorates. Resulting in.

Mo: 0.02 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: One or more of 0.05 to 1.0% These elements are useful elements for improving the strength without degrading the formability by solid solution strengthening. The addition of less than 0.05% for each element cannot effectively exhibit the above-described effect, while the addition of more than 1.0% for each element results in too high a cost.

Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
These elements are useful elements for improving stretch flangeability by making inclusions finer and reducing the starting point of fracture. If less than 0.0005% of each element is added, the above effect cannot be exhibited effectively. On the other hand, if more than 0.01% of each element is added, inclusions are coarsened and stretch flangeability is lowered. To do.

  Next, preferable production conditions for obtaining the steel sheet of the present invention will be described below.

[Preferred production method of the steel sheet of the present invention]
In order to manufacture the cold-rolled steel sheet as described above, first, steel having the above composition is melted and formed into a slab by ingot forming or continuous casting and then hot-rolled.

[Hot rolling conditions]
As hot rolling conditions, the slab is held at 1200 ° C. or higher for 1200 s or longer, finish rolling finish temperature: 900 ° C. or higher, preferably hot rolled at 1000 ° C. or higher, and up to a coiling temperature of 70 ° C./s or higher. Rapid cooling is performed to form a low-temperature transformation phase that is martensite, bainite, or a mixture thereof, and winding is performed at a winding temperature of 450 ° C. or lower.
The MX-type compound is prevented from being precipitated during hot rolling, and the MX-type compound is finely precipitated in the heating process during the subsequent annealing, thereby increasing the strength by precipitation strengthening. The type compound can be refined without the mold compound being the starting point of fracture, and stretch flangeability can be improved.

<Holding the slab at 1200 ° C or higher for 1200s or longer>
Unless maintained at a high temperature before rolling, the MX type compound remains as an undissolved precipitate as a precipitate, and the precipitate serves as a starting point of fracture, and the stretch flangeability deteriorates.

<Finish rolling finish temperature: 900 ° C. or higher, preferably 1000 ° C. or higher>
When the finish rolling finish temperature is less than 900 ° C., the MX type compound precipitates during hot rolling, and the precipitate grows and coarsens in the heating process during the subsequent annealing, and the fine precipitate contributing to precipitation strengthening At the same time as the volume ratio decreases, the coarse precipitates become the starting point of fracture, and stretch flangeability deteriorates.

<Rapid cooling at 70 ° C / s or higher to coiling temperature>
After the finish rolling, when the cooling rate to the coiling temperature is less than 70 ° C./s, ferrite transformation occurs during cooling, precipitates are formed in the formed ferrite, and in the heating process during the subsequent annealing The precipitate becomes coarse and the volume fraction of the fine precipitate that contributes to precipitation strengthening decreases. At the same time, the coarse precipitate becomes a starting point of fracture, and the stretch flangeability deteriorates.

<Taking-up temperature: 450 ° C. or less>
When the coiling temperature exceeds 450 ° C., precipitates are formed or coarsened during winding, and the volume fraction of fine precipitates contributing to precipitation strengthening is reduced, and at the same time, the coarsened precipitates become the starting point of fracture, Stretch flangeability deteriorates.

  After hot rolling is finished, pickling is performed and then cold rolling is performed. The cold rolling rate (hereinafter also referred to as “cold rolling rate”) is preferably about 30% or more. After the cold rolling, annealing and tempering are subsequently performed.

[Annealing conditions]
As annealing conditions, the temperature range of 550 to 600 ° C. is maintained for 60 s or more and 300 s or less, the temperature range of 600 ° C. to Ac ₁ is increased at a rate of 5 ° C./s or more and 30 ° C./s or less, and annealing is performed. _{temperature: [(8 × Ac 1 +} 2 × Ac 3) / 10] in to Ac _3, the annealing retention time: 300 s after holding less, the cooling rate of more than 50 ° C. / s to a temperature of Ms point or lower from the annealing heating temperature Cool quickly.

<Holding for 60s to 300s in the temperature range of 550 to 600 ° C>
This is because the MX type compound is precipitated uniformly and finely before recrystallization of the ferrite, and the ferrite grains are refined during the subsequent recrystallization. If the residence time is 60 s or less, fine precipitation does not occur sufficiently, and ferrite grains cannot be refined during recrystallization. On the other hand, when the staying time exceeds 300 s, the coarsening of the precipitate proceeds and the desired strength and stretch flangeability cannot be obtained.

<Temperature rise from 600 ° C. to Ac _{1 at} a rate of 5 ° C./s to 30 ° C./s>
This is to promote recovery and recrystallization of the ferrite before reverse transformation and release the strain in the ferrite. When the temperature rising rate is less than 5 ° C./s, the MX type compound is coarsened and the desired strength and stretch flangeability cannot be obtained. Further, at a heating rate of more than 30 ° C./s, the ferrite recovery / recrystallization does not proceed sufficiently and the desired elongation cannot be obtained.

<Annealing heating _{temperature: [(8 × Ac 1 +} 2 × Ac 3) / 10] in to Ac _3, the annealing retention time: 300 s or less retained>
This is because a region having an area ratio of 20% or more is transformed into austenite during annealing and thereby a sufficient amount of the hard second phase is transformed during cooling.

When the annealing heating temperature is less than [(8 × Ac ₁ + 2 × Ac ₃ ) / 10], the amount of transformation to austenite is insufficient during annealing and heating, and the amount of hard second phase that is transformed from austenite during subsequent cooling. On the other hand, when the heating exceeds Ac ₃ , the ferrite phase disappears and elongation cannot be obtained.

  Further, when the annealing holding time exceeds 300 s, the MX type compound is coarsened, and a sufficient precipitation strengthening effect cannot be obtained. At the same time, the coarsened precipitate becomes a starting point of fracture, and the stretch flangeability deteriorates. .

A preferable upper limit of the annealing heating temperature is [(1 × Ac ₁ + 9 × Ac ₃ ) / 10] ° C. In the annealing heating stage, the temperature is closer to Ac ₃ point than Ac ₁ point, and when the austenite ratio is increased, the ferrite becomes a structure surrounded by austenite. Therefore, the final structure is preferably the ferrite surrounded by the hard second phase. Become an organization.

<Rapid cooling at a cooling rate of 50 ° C./s or higher to a temperature below the Ms point>
This is to suppress the formation of ferrite from austenite during cooling and obtain a hard second phase.

  When quenching is terminated at a temperature higher than the Ms point or when the cooling rate is less than 50 ° C./s, bainite is formed, and the strength of the steel sheet cannot be secured.

[Tempering conditions]
As tempering conditions, the temperature after annealing cooling to the tempering heating temperature: between 420 ° C. and less than 670 ° C. is heated at a heating rate of more than 5 ° C./s, and [tempering heating temperature−10 ° C.] to tempering heating temperature. Time (tempering holding time) existing in the temperature region between: After 20 s or less, it may be cooled at a cooling rate of more than 5 ° C./s.

  The rate of strain reduction in ferrite and hard second phase is strongly temperature dependent, while the size of cementite particles is time dependent. Therefore, it is effective to increase the tempering heating temperature and shorten the tempering holding time in order to suppress the cementite coarsening while releasing the strain.

<The temperature after the annealing cooling to the tempering heating temperature: between 420 ° C. and less than 670 ° C. is heated at a heating rate exceeding 5 ° C./s>
When the heating rate is 5 ° C./s or less, nucleation / growth of cementite occurs during heating, coarse cementite is formed, and stretch flangeability cannot be secured.

<Tempering heating temperature: 420 ° C. or more and less than 670 ° C., tempering holding time: 20 s or less>
When the tempering heating temperature is less than 420 ° C., the strain in the ferrite or hard second phase is not sufficiently released, and elongation and stretch flangeability cannot be secured. On the other hand, when the tempering heating temperature is 670 ° C. or higher or the tempering holding time exceeds 20 s, the strength of the hard second phase is insufficient and the cementite and MX type compounds are coarsened, and the strength and stretch flangeability cannot be secured. .
A preferable range of the tempering heating temperature is 450 ° C. or more and less than 650 ° C., a more preferable range is 450 ° C. or more and less than 600 ° C., and a preferable range of the tempering holding time is 10 s or less, more preferably 5 s or less.

  Steels having the components shown in Table 1 below were melted to produce 120 mm thick ingots. This was hot rolled to a thickness of 25 mm, and then hot rolled again to a thickness of 3.2 mm under the hot rolling conditions shown in Table 2 below. After pickling, this was cold-rolled to a thickness of 1.6 mm to obtain a test material, which was heat-treated under the annealing conditions and tempering conditions shown in Table 2.

  For each steel plate after the heat treatment, the area ratio of each phase, the average grain size and existence form of ferrite, and the sizes of cementite and MX type compound were measured according to the measurement method described in the above [Mode for Carrying Out the Invention]. And its abundance was measured.

  Moreover, about each said steel plate, tensile strength TS, elongation El, and stretch flangeability (lambda) were measured. The tensile strength TS and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis perpendicular to the rolling direction. Moreover, stretch flangeability (lambda) measured the hole expansion rate by implementing the hole expansion test in accordance with the iron standard JFST1001, and this was made into the stretch flangeability.

The measurement results are shown in Tables 3 and 4.

  As shown in Table 3 and Table 4, No. 1 to 5, 15, 19 to 23, 33, 36 to 38, 40, 41, 43, 45 to 49, all have a tensile strength of 980 MPa or more, TS × El is 17000 MPa ·% or more, and TS × El A high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability satisfying the required level described in the above [Background Technology] was obtained, with xλ satisfying 1000000 MPa ·% ·% or more.

  On the other hand, the comparative example No. 6-14, 16-18, 24-32, 34, 35, 39, 42, 44 are inferior in at least one of TS, TS × E1 and TS × E1 × λ.

For example, no. 6 to 9, 12 to 14, 24 to 27, and 30 to 32 are at least one of the requirements for defining the structure of the present invention because the hot rolling condition, annealing condition or tempering condition is out of the recommended range. Is not satisfied and at least one of TS, TS × E1 and TS × E1 × λ is inferior. No. 10, 11, 28, and 29 are inferior in at least one of TS, TS × El, and TS × El × λ because the hot rolling conditions, annealing conditions, or tempering conditions are outside the recommended range.

  No. Since 16 and 34 do not contain the microalloy (V, Ti, Nb) forming the MX type compound, the effect of refining ferrite and the effect of precipitation strengthening cannot be obtained, and TS × El is inferior.

  No. Since Nos. 17 and 35 exceeded the upper limit of the amount of Ti or Nb added, a coarse MX type compound was formed, and the stretch flangeability deteriorated. Therefore, TS × El × λ was inferior.

  No. No. 18 is inferior in TS, TS × El, and TS × El × λ because the C content is too low.

  No. In No. 39, when the Si content is too high, both elongation and stretch flangeability deteriorate, and TS × El and TS × El × λ are inferior.

  No. In No. 42, since the Mn content is too high, the reverse transformation temperature becomes too low and recrystallization cannot be performed, so that both elongation and stretch flangeability deteriorate, and TS × El and TS × El × λ are inferior.

  No. No. 44 is inferior in TS × E1 and TS × E1 × λ because the cementite particles and MX type compounds that are coarsened due to the C content being too high.

Claims (5)

% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Si: 3.0% or less (including 0%),
Mn: 0.1 to 5.0%,
P: 0.1% or less (including 0%),
S: 0.010% or less (including 0%),
Al: 0.001 to 0.10%,
V: 0.10 to 0.50%
In order to satisfy the requirements, and the remainder has a component composition consisting of iron and inevitable impurities,
In addition to containing ferrite, which is a soft first phase, in an area ratio of 10 to 80%, the residual austenite, martensite, and the mixed structure of residual austenite and martensite are less than 5% (including 0%) in total area ratio There, the balance being a hard second phase, has a structure consisting of martensite and / or tempering bainite tempering,
The average particle diameter of the ferrite is 2 μm or less in terms of equivalent circle diameter,
The distribution state of precipitates present in the hard second phase in contact with the ferrite is
There are 3 or less cementite particles having an equivalent circle diameter of 0.1 μm or more per 1 μm ^{2 of the} hard second phase,
Of the particles containing V present in the form of precipitates in the ferrite, particles having an equivalent circle diameter of 8 nm or more and less than 20 nm account for 60% or more on a number basis with respect to the total number of particles containing V.
And, among the particles comprising the V present in the form of precipitates in the ferrite and the hard second phase, a circle equivalent diameter 20nm or more of the particles, are three or der less per 1 [mu] m ^2,
The tensile strength TS is 980 MPa or more, the product TS × El of the tensile strength TS and the elongation El is 17000 MPa ·% or more, and the product TS × El × λ of the tensile strength TS, the elongation El and the stretch flangeability λ is 1000000 MPa ·%. % High-strength cold-rolled steel sheet with an excellent balance between elongation and stretch flangeability. The component composition further includes Ti and / or Nb so as to simultaneously satisfy the following formulas 1 to 3,
Of the particles containing one or more of V, Ti and Nb present in the form of precipitates in the ferrite, particles having an equivalent circle diameter of 8 nm or more and less than 20 nm are 1 of V, Ti and Nb. Occupies 60% or more on a number basis with respect to the total number of particles containing two or more species,
Of the particles containing one or more of V, Ti and Nb present in the form of precipitates in the ferrite and the hard second phase, particles having an equivalent circle diameter of 20 nm or more are 3 per 1 μm ^2. Less than
A high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1.
Formula 1: log ([% Ti] × [% C]) <− 1.6
Formula 2: log ([% Nb] × [% C]) <− 2.0
Formula 3: ([% V] / 51 + [% Ti] / 48 + [% Nb] / 93) × 51 ≦ 0.50%
Here, [% X] means the content (mass%) of the element X. Ingredient composition further
Cr: 0.01 to 1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2. Ingredient composition further
Mo: 0.02 to 1.0%,
Cu: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to any one of claims 1 to 3, wherein Ni: one or more of 0.05 to 1.0% is included. Ingredient composition further
Ca: 0.0005 to 0.01%, and / or
Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to any one of claims 1 to 4.