JP2010255090A - High strength cold-rolled steel sheet having excellent balance between elongation and stretch-flangeability, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength cold-rolled steel sheet having an improved balance between elongation and stretch-flangeability, and better formability, and to provide a method for producing the same. <P>SOLUTION: The cold-rolled steel sheet has a composition containing, by mass, 0.05-0.30% C, ≤3.0% (including 0%) Si, 0.1-5.0% Mn, ≤0.1% (including 0%) P, ≤0.010% (including 0%) S, and 0.001-0.10% Al, and the balance mainly iron, and has a structure comprising 10-80% ferrite, <5% (including 0%) the sum of retained austenite and martensite, by an area rate, and the balance a hard second phase, wherein for a KAM value frequency distribution curve, the relationship between the proportion X<SB>KAM≤0.4°</SB>of sites having a KAM value≤0.4° and the areal ferrite proportion V<SB>α</SB>satisfies X<SB>KAM≤0.4°</SB>/V<SB>α</SB>≥0.8 and the proportion X<SB>KAM=0.6-0.8°</SB>of sites having a KAM value in the range of 0.6-0.8 is 10-20%. In the hard second phase adjoining the ferrite, three or less cementite particles having an equivalent-circle diameter of 0.1 μm or larger are dispersed per μm<SP>2</SP>of the hard second phase. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a high-strength steel sheet excellent in workability used for automobile parts and the like and a method for producing the same, and more specifically, a high-strength steel sheet having an improved balance between elongation (total elongation) and stretch flangeability and a method for producing the same. About.

  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 780 MPa or higher and an improved balance between elongation (total elongation; El) and stretch flangeability (hole expansion ratio; λ) is eagerly desired. For example, the tensile strength TS is 780 MPa or more, TS × El is 14000 MPa ·% or more, and TS × El × λ is 800,000 MPa ·% ·% or more (more preferably, the tensile strength TS is 780 MPa or more, TS × El is 15000 MPa ·% or more and TS × El × λ is 1000000 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 that contains at least one of Mn, Cr, and Mo in a total amount of 1.6 to 2.5% by mass and is substantially composed of a single-phase structure of martensite. In a 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-mentioned required level is satisfied. (See the invention example in Table 6 of the same document).

  Patent Document 2 discloses a high-tensile steel sheet 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 plate 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%. .

  All of the high-tensile steel sheets disclosed in Patent Documents 2 and 3 have an elongation exceeding 10% by mixing a large amount of highly deformable ferrite, and there are some that satisfy the above-mentioned required level ( (See the invention examples in Table 2 of Patent Document 2 and the examples in Table 2 of Patent Document 3). However, although the invention relating to these high-tensile steel sheets is characterized by controlling the area ratio between the ferrite and the hard second phase, and further controlling the grain size of these two phases, the strain amount in the ferrite, the hard second phase The present invention clearly differs from the technical idea of the present invention, which is characterized by the control of the deformability and the distribution state of cementite particles present at the interface between the ferrite and the hard second phase.

JP 2002-161336 A JP 2004-256872 A JP 2004-232022 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%
And the remainder has a component composition consisting of iron and inevitable impurities,
While containing 10-80% of area ratio of ferrite which is a soft first phase,
Including a retained austenite, martensite, and a mixed structure of retained austenite and martensite in a total area ratio of less than 5% (including 0%),
The balance is a hard second phase, and has a structure composed of tempered martensite and / or tempered bainite,
In the frequency distribution curve of the Kernel Average Misoration value (hereinafter abbreviated as “KAM value”),
The relationship between the ratio X _{KAM ≦ 0.4 °} (unit:%) of the frequency at which the KAM value is 0.4 ° or less to the total frequency and the ferrite area ratio V _α (unit:%) is _expressed by X _{KAM ≦ 0.4 °} / V _α ≧ 0.8
The ratio X _{KAM = 0.6-0.8 °} of the frequency with the KAM value of 0.6-0.8 to the total frequency is 10-20%, and
Elongation characterized in that the dispersion state of cementite particles having an equivalent circle diameter of 0.1 μm or more present in the hard second phase in contact with the ferrite is 3 or less per 1 μm ² of the hard second phase; It is a high-strength cold-rolled steel sheet with an excellent balance of stretch flangeability.

The invention described in claim 2
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.

The invention according to claim 3
Ingredient composition further
Mo: 0.02-1.0%
Cu: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet excellent in the balance between elongation and stretch flangeability according to claim 1 or 2, comprising Ni: 0.05 to 1.0%, or one or more.

The invention according to claim 4
Furthermore,
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 3.

The invention described in claim 5
A steel material having the composition shown in any one of claims 1 to 4 is hot-rolled under the conditions shown in the following (1) to (4), then cold-rolled, and then annealed. It is a method for producing a high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability, characterized by tempering.
(1) Hot rolling conditions Finish rolling end temperature: Ar ₃ points or more Winding temperature: 450-700 ° C
(2) Cold rolling conditions Cold rolling rate: 20-80%
(3) Annealing conditions The temperature range of 600 to Ac1 ° C. is raised in a temperature rising pattern that satisfies both the following formulas 1 and 2, and the annealing heating temperature: [(8 × Ac1 + 2 × Ac3) / 10] to 1000 ° C. In the annealing holding time: after holding at 3600 s or less, quench from the annealing heating temperature directly to the temperature below the Ms point at a cooling rate of 50 ° C./s or from the annealing heating temperature to below 600 at the annealing heating temperature. After cooling slowly at a cooling rate (referred to as “first cooling rate”) of 1 ° C./s or more and less than 50 ° C./s until a temperature equal to or higher than 0 ° C. (referred to as “first cooling end temperature”), a temperature below the Ms point (It is referred to as “second cooling end temperature”) and is rapidly cooled at a cooling rate of 50 ° C./s or less (referred to as “second cooling rate”).
(4) Tempering conditions 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 of 5 ° C./s, and [tempering heating temperature−10 ° C.] to tempering heating temperature. Time existing in the temperature region between (referred to as “tempering holding time”): 30 seconds or less, and then cooled at a cooling rate of more than 5 ° C./s.

  According to the present invention, in the dual phase structure steel mainly composed of ferrite as a soft first phase and tempered martensite and / or tempered bainite as a hard second phase, the amount of strain in the ferrite is controlled, By introducing an appropriate amount of hard second phase with high deformability and further controlling the distribution of cementite particles present at the interface between ferrite and hard second phase, it is possible to improve stretch flangeability while ensuring elongation. This has made it possible to provide a high-strength steel sheet with a better balance between elongation and stretch-flangeability and with better formability.

It is a graph which shows frequency distribution of a KAM value.

  The inventors of the present invention mainly include a composite phase composed of ferrite, which is a soft first phase, and tempered martensite and / or tempered bainite (hereinafter, sometimes referred to as “tempered martensite”) which is a hard second phase. Focusing on the high-strength steel sheet with a structure, if the stretch flangeability can be improved while securing the elongation, it is considered that a high-strength steel sheet that can satisfy the above-mentioned desired level can be obtained, and 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 the amount of strain in the ferrite is controlled, the deformability of the hard second phase is controlled, and the cementite precipitated at the interface between the ferrite and the hard second phase is further refined. Thus, 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 those of Patent Documents 2 and 3 above. In particular, while controlling the amount of strain in ferrite and the deformability of the hard second phase, It is different from the steel sheets of Patent Documents 2 and 3 in that it is controlled and the distribution state of cementite particles precipitated at the interface between the ferrite and the hard second phase 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 if 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, if 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 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 grain boundaries”) / (number of intersections with ferrite grain boundaries) + “ferrite-hard second phase interface” The number of intersections ”) is 0.5 or less.

<Retained austenite, martensite, and mixed structure of retained austenite and martensite: less than 5% (including 0%) in total area ratio,
Remainder: second phase, tempered martensite and / or tempered bainite>
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 action, the residual austenite, martensite, and the 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 tempered martensite and / or tempered bainite.

<Relationship between ratio X _{KAM ≦ 0.4 °} with KAM value of 0.4 ° or less and ferrite area ratio V _α : X _{KAM ≦ 0.4 °} / V _α ≧ 0.8,
Ratio of KAM value 0.6 to 0.8 ° X _{KAM = 0.6 to 0.8 °} : 10 to 20%>
The balance between the strength and elongation of the dual phase structure steel generally depends on the ferrite area ratio and the deformability of the hard second phase. On the other hand, the amount of strain in the ferrite has a large effect on the elongation. When the ferrite area ratio is constant, the elongation decreases if the amount of strain is large.

  When considering only the balance between strength and elongation, the decrease in elongation due to the presence of strain in ferrite improves the elongation by increasing the ferrite area ratio and decreases the degree of tempering of the hard second phase. As a result, the balance between strength and elongation can be secured.

  However, when considering the stretch flangeability in addition to the strength and elongation, as the strain remains in the ferrite as described above, the ferrite area ratio is increased to ensure a balance between the elongation and the strength. It was found that when the treatment of increasing the strength of the two phases is performed, the deformability of the hard second phase is reduced, so that strain concentrates on the interface between the ferrite and the hard second phase, and the stretch flangeability deteriorates.

  From this finding, if the amount of strain in the ferrite is reduced as much as possible, it is possible to increase the deformability of the hard second phase by reducing the ferrite area ratio necessary to secure the same balance between strength and elongation. It has been found that the stretch flangeability can be improved, and as a result, the balance between strength, stretch and stretch flangeability can be improved.

  That is, in order to ensure elongation and stretch flangeability while ensuring a constant strength, it is important to reduce the amount of strain in ferrite and to increase the deformability of the hard second phase.

  It is effective to use the KAM value for evaluating the strain amount in ferrite and the deformability of the hard second phase.

The KAM value is an average value of the amount of crystal rotation (crystal orientation difference) between the target measurement point and the surrounding measurement points, and when this value is large, it indicates that strain exists in the crystal. FIG. 1 illustrates a KAM value frequency distribution curve obtained by scanning a certain region with a scanning electron microscope for the steel of the present invention. Thus, the KAM value frequency distribution curve shows two peaks. The first peak with a KAM value near 0.2 ° is due to strain in the ferrite, and the second peak with a KAM value near 0.6 ° is due to strain in the hard second phase. As the strain in each phase increases, each peak moves to the high KAM value side. On the other hand, for example, when the area ratio of ferrite increases, the first peak height increases. _Taking these phenomena into consideration, X _{KAM ≦ 0.4 °} / V _α and X _{KAM = 0.6} to _{0.8 °} are used as indices that _simply represent the amount of strain in ferrite and the deformability of the hard second phase _. Each was introduced.

Here, X _{KAM ≦ 0.4 °} is the ratio of the frequency with a KAM value of 0.4 ° or less to the total frequency, V _α is the area ratio of ferrite, and X _{KAM = 0.6 to 0 .8 °} is the ratio of the frequency with a KAM value of 0.6 to 0.8 ° with respect to the total frequency.

X _{KAM ≦ 0.4 °} , that is, the ratio of the frequency with a KAM value of 0.4 ° or less to the total frequency is considered to be a function of the amount of strain in the ferrite and the area ratio of the ferrite. by dividing by the area ratio V _alpha, it is obtained as an index representing the amount of strain in ferrite. As the amount of strain in the ferrite increases, the first peak position moves to the high KAM value side, and X _{KAM ≦ 0.4 °} / V _α decreases.

In order to minimize the amount of strain in the ferrite, X _{KAM ≦ 0.4 °} / V _α is set to 0.8 or more (preferably 0.9 or more, more preferably 1.1 or more). In other words, if X _{KAM ≦ 0.4 °} is 30% or more, it means that 20% or more of ferrite with a small strain exists.

Further, X _{KAM = 0.6 to 0.8 °} , that is, the ratio of the frequency of the KAM value of 0.6 to 0.8 with respect to the total frequency indicates the amount of the hard second phase having high deformability. If this ratio is 10% or more, the amount of the hard second phase and the deformability are sufficient to ensure a balance between strength, elongation and stretch flangeability. On the other hand, if the ratio exceeds 20%, the amount of the hard second phase becomes too large, so that elongation cannot be secured.

A preferable range of X _{KAM = 0.6 to 0.8 °} is 12 to 18%, and a more preferable range is 13 to 16%.

<Dispersion state of cementite particles having an equivalent circle diameter of 0.1 μm or more present in the hard second phase in contact with the ferrite interface: 3 or less per 1 μm ² of the hard second phase>
When the fracture at the interface between the ferrite and the hard second phase can be suppressed by satisfying the requirements regarding the KAM value as described above, the next starting point of the fracture is in the hard second phase contacting the ferrite and the interface It becomes cementite deposited on the surface. If the cementite particles become 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.

  Hereinafter, a method for measuring the area ratio of each phase, the KAM value, the size and density of cementite particles, and the existence form of ferrite will be described.

[Measurement method 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. And the area ratio of each phase was computed from the area ratio of each area | region.

[Measurement method of KAM value]
Each test steel sheet was mirror-polished and further electropolished, and then an electron beam backscatter diffraction image in a 500 μm × 500 μm region was measured at 1 step 0.2 μm with a scanning electron microscope (Philips XL30S-FEG). The KAM value at each measurement point was determined using analysis software (Osem system manufactured by Tecsem Laboratories).

[Method of measuring the size and density of cementite particles]
About the size of cementite particles and the density of their existence, 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 3 fields of 2.4 μm × 1.6 μm. From the contrast of the image, the white portion is marked as cementite particles and marked, and the image analysis software calculates the equivalent circle diameter D (D = 2 × (A / π) ^1/2 from the area A of each marked cementite particle. ) And the number of cementite particles of a predetermined size present per unit area. A portion where a plurality of cementite particles overlap was excluded from the observation target.

[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. Then, the ratio N _α / (N _α + N _α−TM ) of the ferrite grain boundary occupying the grain boundary and the interface is obtained as an evaluation index of the existence form of the ferrite. 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%, in addition to a large amount of distortion during quenching, the amount of cementite increases and the dislocations are difficult to recover, so that the dislocation is lost and the hard second phase has improved deformability. X _{KAM = 0.6 to 0.8 °} ≧ 10%, which is an evaluation formula indicating the above, cannot be obtained. If the tempering conditions are increased to a high temperature or a long time so as to satisfy this evaluation formula, the cementite becomes coarse, and the strength and stretch flangeability cannot be secured.

  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. If it exceeds 3.0%, the formation of austenite at the time of heating is inhibited, so that the area ratio of the hard second phase cannot be ensured and stretch flangeability cannot be ensured. 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 contributes to both elongation and stretch flangeability by increasing the deformability of the hard second phase, in addition to having the effect of suppressing coarsening of cementite during tempering, similar to Si. 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 0.50 to 2.5%, more preferably 1.2 to 2.2%.

P: 0.1% or less P is inevitably 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% or less, 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 cracks 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 element and lowers the elongation and stretch flangeability by strain aging, so the lower one is preferable, and the content is made 0.01% or less.

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%, solute 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%, the formation of austenite during heating is inhibited. The area ratio of the hard second phase cannot be secured, and the stretch flangeability cannot be secured.

  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.

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: 0.02-1.0%
Cu: 0.05 to 1.0%,
Ni: One or more of 0.05 to 1.0% These elements are useful elements for improving strength without degrading 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 miniaturizing inclusions 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, the preferable manufacturing method for obtaining this invention steel plate is demonstrated below.

[Preferred production method of the steel sheet of the present invention (part 1)]
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. As hot rolling conditions, the finishing temperature of finish rolling is set to Ar ₃ point or higher, and after appropriate cooling, winding is performed in a range of 450 to 700 ° C. 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.

  Then, after the cold rolling, annealing and further tempering are performed.

[Annealing conditions]
As annealing conditions, the temperature range of 600 to Ac1 ° C. was raised with a stay time of (Ac1-600) s or more, and the annealing heating temperature: [(8 × Ac1 + 2 × Ac3) / 10] to 1000 ° C. was annealed. Holding time: After holding for 3600 s or less, quench immediately from the annealing heating temperature to a temperature below the Ms point at a cooling rate of 50 ° C./s or from the annealing heating temperature to a temperature of 600 ° C. or more below the annealing heating temperature. After gradually cooling at a cooling rate (first cooling rate) of 1 ° C./s or more and less than 50 ° C./s to (first cooling end temperature), 50 ° C./s to a temperature below the Ms point (second cooling end temperature) It is preferable to rapidly cool at the following cooling rate (second cooling rate).

<Temperature rise in the temperature range of 600 to Ac1 ° C. with a stay time of (Ac1-600) s>
This is to promote the recovery and recrystallization of ferrite by allowing them to stay in a high temperature region for a long time before reverse transformation, thereby releasing the strain in the ferrite.
It is preferable to raise the temperature range of 600 to Ac1 ° C. with a residence time of 200 s or more, and more preferably with a residence time of 1000 s or more.

<Annealing heating temperature: [(8 × Ac1 + 2 × Ac3) / 10] to 1000 ° C., annealing holding time: 3600 s or less>
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 × Ac1 + 2 × Ac3) / 10] ° C., the amount of transformation to austenite is insufficient during annealing heating, so that the amount of hard second phase that transforms from austenite during the subsequent cooling is ensured. On the other hand, heating exceeding 1000 ° C. is industrially difficult with existing annealing equipment.

  Further, if the annealing holding time exceeds 3600 s, productivity is extremely deteriorated, which is not preferable.

  A preferable upper limit of the annealing heating temperature is [(1 × Ac1 + 9 × Ac3) / 10] ° C. When a mixed structure of ferrite and austenite is formed in the annealing heating stage, since the ferrite becomes a structure surrounded by austenite, the final structure becomes a preferable structure in which ferrite is surrounded by the hard second phase.

  A preferable lower limit of the annealing heating holding time is 60 s. By increasing the heating time, strain in the ferrite can be further removed.

<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 the rapid cooling is finished 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.

<Slow cooling at a cooling rate of 1 ° C./s or more and less than 50 ° C./s to a temperature of 600 ° C. or more below the heating temperature>
This is because by forming a ferrite structure having an area ratio of less than 50%, the elongation can be improved while the stretch flangeability is secured.

  When the temperature is less than 600 ° C. or the cooling rate is less than 1 ° C./s, ferrite is excessively formed, and the strength and stretch flangeability 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. The time existing in the temperature region between (tempering holding time): after 30 seconds or less, it may be cooled at a cooling rate exceeding 5 ° C./s.

  The rate of strain (dislocation) reduction in ferrite and hard second phase depends strongly on temperature, while the size of cementite particles depends on time. Therefore, in order to reduce the dislocation while releasing the strain, it is effective to increase the tempering temperature and shorten the staying time.

  When the heating rate or cooling rate is 5 ° C./s or less, nucleation / growth of cementite occurs during heating or cooling, and coarse cementite is formed, so that stretch flangeability cannot be secured.

  If the tempering heating temperature is less than 420 ° C., the strain in the ferrite or hard second phase is large, and it becomes impossible to ensure elongation and stretch flangeability.

  On the other hand, when the tempering heating temperature is 670 ° C. or higher or the tempering holding time exceeds 30 s, the strength of the hard second phase is insufficient, and the strength of the steel sheet cannot be secured, or cementite is coarsened and stretch flangeability deteriorates. .

  A preferable range of the tempering heating temperature is 450 ° C. or more and less than 650 ° C., a more preferable range is 500 ° C. or more and less than 600 ° C., a preferable range of the tempering holding time is 10 s or less, and a further preferable range is 5 s or less.

[Preferred production method of the steel sheet of the present invention (part 2)]
In the above [Preferred production method of the steel sheet of the present invention (part 1)], the [annealing condition] stipulates that “the temperature range of 600 to Ac1 ° C. is raised in the stay time of (Ac1-600) s”. It is more preferable to raise the temperature in the temperature range of 600 to Ac1 ° C. with a temperature raising pattern that satisfies both the following formulas 1 and 2. The other production conditions are the same as the above [Preferred production method of the steel sheet of the present invention (Part 1)] (however, the cold rolling rate in the cold rolling is the same as that of the preferred production method of the steel sheet of the present invention (part 1). 1)], “it should be about 30% or more”, but in this example, it is in the range of 20 to 80%, which is the range in which Expression 4 representing the relationship with the initial dislocation density described later is satisfied.

  That is, the present inventors promote the recovery and recrystallization of ferrite by annealing for a long time before reverse transformation during annealing in the above-mentioned [Preferred production method of steel sheet of the present invention (part 1)]. For the purpose of releasing the strain in the ferrite, “the temperature range of 600 to Ac1 ° C. was raised with a stay time of (Ac1-600) s or more”.

  However, according to the subsequent studies by the present inventors, cementite precipitated during cooling after melting of the steel material or cooling after hot rolling may remain in the structure of the steel sheet before annealing. Yes, the cementite remaining in this steel sheet structure becomes coarse when the temperature rises during annealing, and this coarsened cementite is brought in until after tempering, which may deteriorate the stretch flangeability of the steel sheet after heat treatment. I understood it.

  For this reason, more preferable annealing conditions are not only to promote the recovery and recrystallization of ferrite, but also to prevent the coarsening of cementite remaining in the structure of the steel sheet before annealing, while recovering and recrystallizing ferrite. We thought that it was necessary to adopt a temperature rising pattern that promoted

  Therefore, in order to accurately determine such a temperature rising pattern, the recrystallization rate X is used as an index that quantitatively represents the degree of ferrite recovery and recrystallization, and cementite is used as an index that quantitatively represents the degree of cementite coarsening. The particle radius r was adopted, and first, the effects of the treatment temperature and treatment time on these indices were investigated.

Here, the recrystallization rate X is expressed by the following formula 1 ′ as a result of examining the effects of the recrystallization temperature and time using a material in which the initial dislocation density ρ ₀ is changed by changing the cold rolling rate. I found out that I can.
Formula 1 ′: X = 1−exp [−exp {A ₁ ln (D _Fe ) + A ₂ ln (ρ ₀ ) −A ₃ } · t ⁿ ] (where A ₁ , A ₂ , A ₃ , n: constant)

And the iron self-diffusion rate D _Fe is
Formula 3: D _Fe = 0.0118exp [-281500 / {R (T + 273)}] (m ² / s) (where T: temperature (° C.), R: gas constant [= 8.314 kJ / (K · kg-atom)]) is known to hold (see, for example, Japan Iron and Steel Institute, Steel Handbook 3rd Edition, I Fundamentals, Maruzen, 1981, p.349).

In addition, for the initial dislocation density ρ ₀ , the correlation between the initial dislocation density ρ ₀ and the cold rolling rate [CR] was investigated using steel sheets cold-rolled at 20 to 80% on various steel materials. As a result, it turned out that it can represent with the following formula 4. The dislocation density was measured using the method disclosed in Japanese Patent Application Laid-Open No. 2008-144233.
Formula 4: ρ ₀ = B ₁ ln [(− ln {(100− [CR]) / 100}] + B ₂ (B ₁ , B ₂ : constant)

As a result of determining the values of the constants B ₁ and B ₂ of the above formula 4 based on the above investigation results, B ₁ = 1.54 × 10 ¹⁵ , B _{2 in} the range of the cold rolling rate [CR]: 20 to 80%. = 2.51 × 10 ¹⁴ was obtained.

On the other hand, the cementite particle radius r is known to grow according to the cube law of r, and can be simply written down as in the following equation 2 ′ (for example, Kento Sakuma, The Japan Institute of Metals, No. 1 20 volume, 1981, p.247).
Formula 2 ′: r ³ −r ₀ ³ = A · exp [−Q / {R (T + 273)}] · t (where A and Q are constants)

  And in order to determine the value of each constant in each said relational expression, the following tests were implemented.

  C: 0.17%, Si: 1.35%, Mn: 2.0%, which is within the range of the composition of the present invention, remains cold-rolled at a cold rolling rate of 36% (annealing)・ Two kinds of cold rolled steel sheets (before tempering) and actual cold rolled steel sheets (thickness 1.6 mm) and cold rolled steel sheets with a cold rolling rate of 36% and cold rolled to a cold rolling rate of 60% Was used as a test material.

  Then, the two types of cold-rolled steel plates are heat-treated with various combinations of holding temperatures and holding times in a heat pattern of “rapid heating + holding at a constant temperature for a predetermined time + rapid cooling” to harden the steel plates before and after the heat treatment. It is considered that there is a strong correlation between the change in hardness and the recrystallization rate, so the recrystallization rate = (hardness before heat treatment−hardness after heat treatment) / (hardness before heat treatment). The recrystallization rate was calculated by the definition formula of (−180Hv). Here, 180Hv in the above definition formula is the lowest hardness that does not soften any more when heat treatment is performed by sequentially extending the holding time in the state where the holding temperature is the highest, and it is sufficiently annealed and re-applied. This corresponds to the hardness of the state where crystallization is completed and completely softened.

The data of the recrystallization rate X calculated in this way is subjected to an Abrami plot as a relationship between the holding temperature T and the holding time t, whereby constants A ₁ , A ₂ , A in the above formula 1 ′ are obtained. As a result of determining the values of ₃ and n, A ₁ = 0.8, A ₂ = 1.8, A ₃ = 33.7, and n = 0.58 were obtained.

Further, for the two types of cold-rolled steel sheets, the average radii r ₀ and r of cementite particles existing in the steel sheet structure before and after the heat treatment performed with various combinations of holding temperature T and holding time t were measured, respectively (r ³ −r ₀ ³ ) / t is set to 1 / T, and the values of the constants A and Q in the expression 2 ′ are determined by Arrhenius plot (Arhenius Plot). As a result, A = 0.5, Q = 80220 was obtained.

  Since the above formulas 1 ′ and 2 ′ are formulas when T is constant, these formulas are changed to a temperature T (t) as a function of time t so that they can be applied to the temperature raising process. Then, Equation 1 and Equation 2 were derived by transforming into a form that integrates with a residence time between 600 and Ac1 ° C.

  And about the steel plate heat-processed on various annealing conditions, it calculated using the formula 1 and formula 2 which were derived | led-out as mentioned above, and the structure | tissue of the steel plate after the actual heat processing calculated using the recrystallization rate X and the cementite particle radius r When the recrystallized state confirmed by observation was compared with the coarsened state of cementite, good agreement was found between the two, and the recrystallization rate X and cementite according to these formulas 1 and 2 were found. It was confirmed that the prediction accuracy of the particle radius r was sufficiently high.

  In addition, the relationship between the recrystallization ratio X and the cementite particle radius r calculated using the formulas 1 and 2 and the mechanical properties of the steel sheet after the heat treatment (annealing + tempering) was investigated. From the results of the investigation, as a more preferable annealing condition, the value of TS × E1 × λ of the steel sheet after the heat treatment is the desired level (800,000 MPa ·% ·% or more, more preferably 1,000,000 MPa · As a result of obtaining a combination of X and r that is more than 1500,000 MPa ·% ·%, which is higher than (% ·% or more), X ≧ 0.8 and r ≦ 0.19 were obtained.

  In other words, by adopting a temperature rising pattern during annealing that satisfies both X ≧ 0.8 and r ≦ 0.19, both the recovery of ferrite and the promotion of recrystallization and the prevention of coarsening of cementite are achieved. In addition, a steel sheet having an excellent balance of mechanical properties can be obtained.

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. After pickling this, it cold-rolled to 1.6 mm in thickness to make a test material, and heat-treated on the conditions shown in Table 2 and Table 3.

  Here, Steel No. 1-32, 36 were heated from 600 ° C. to T1 (° C.) (however, 600 ° C. <T1 <Ac1) as a temperature rising pattern from 600 ° C. to Ac1 during annealing at a predetermined temperature rising rate. Thereafter, it is held for a certain time at T1, and thereafter, T1 to Ac1 are heated at a predetermined temperature increase rate.

  On the other hand, Steel No. Nos. 33 to 35 and 37 were heated from 600 ° C. to T1 (° C.) (however, 600 ° C. <T1 <Ac1) as a temperature rising pattern from 600 ° C. to Ac1 during the annealing. Thereafter, T1 to Ac1 were immediately heated at a predetermined temperature increase rate without holding the temperature at T1 ° C.

In addition, Ac1 and Ac3 in Table 1 are experimentally measured in advance. As a specific measurement method, a sample of φ8 mm × 12 mmL is continuously heated with a heat treatment simulator at 5 ° C./s to measure an expansion curve (relationship between temperature and expansion coefficient), and the inflection point of the expansion curve The temperatures of were set to Ac1 and Ac3.

  About each steel plate after heat treatment, the area ratio of each phase, KAM value, size and number of cementite particles, and the presence form of ferrite by the measurement method described in the above section [Mode for carrying out the invention] 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) performed the hole expansion test according to the iron continuous standard JFST1001, and measured the hole expansion rate, and made this the stretch flangeability.

  The measurement results are shown in Tables 4 and 5.

  As shown in these tables, Steel No. 1, 2, 7, 11, 14, 16-21, 24, 25, 27-33, 35-37, the tensile strength TS is 780 MPa or more, TS × El is 14000 MPa ·% or more, and TS 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] satisfying xEl × λ of 800,000 MPa ·% ·% or more was obtained. .

  Among the above invention examples, in particular, steel No. Nos. 32, 33, 36, and 37 satisfy both X ≧ 0.8 and r ≦ 0.19, which are the recommended conditions of the above-mentioned [Preferred production conditions of the present invention (Part 2)], in the temperature rising pattern during annealing. Therefore, TS × E1 × λ satisfies a level of more than 1500,000 MPa ·% ·%, far exceeding the desired level, and a high-strength cold-rolled steel sheet having an excellent balance of mechanical properties was obtained.

  However, among the above invention examples, the steel No. 35, although the temperature rise pattern during annealing satisfies X ≧ 0.8, r exceeds 0.19, so λ is slightly lower, and TS × El × λ reaches 1500,000 MPa ·% ·%. Not.

On the other hand, steel No. which is a comparative example. 3 to 6, 8 to 10, 12, 13, 15, 22, 23, 26, and 34 are inferior in at least one of TS × El and TS × El × λ.

  For example, steel no. 3, 4, 6, 8 to 10 and 34 do not satisfy at least one of the requirements for defining the structure of the present invention because the annealing condition or tempering condition is out of the recommended range, and TS × El, TS At least one of × E1 × λ is inferior.

  Steel No. In No. 13, since the C content is too low, the area ratio of ferrite becomes too large, so TS × El is inferior.

  On the other hand, Steel No. No. 15 is inferior in TS × E1 × λ because the C content is too high and the amount of coarsened cementite particles increases too much.

  Steel No. 23, since the Mn content is too low, the effect of suppressing cementite coarsening during tempering and the effect of improving the deformability of the hard second phase are not sufficiently exhibited. × El × λ is inferior.

Steel No. In No. 26, since the Mn content is too high, the reverse transformation temperature becomes too low and recrystallization cannot be performed, so the balance between strength and elongation cannot be secured, and both TS × El and TS × El × λ are inferior. .

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%
And the remainder has a component composition consisting of iron and inevitable impurities,
While containing 10-80% of area ratio of ferrite which is a soft first phase,
Including a retained austenite, martensite, and a mixed structure of retained austenite and martensite in a total area ratio of less than 5% (including 0%),
The balance is a hard second phase, and has a structure composed of tempered martensite and / or tempered bainite,
In the frequency distribution curve of the Kernel Average Misoration value (hereinafter abbreviated as “KAM value”),
The relationship between the ratio X _{KAM ≦ 0.4 °} (unit:%) of the frequency at which the KAM value is 0.4 ° or less to the total frequency and the ferrite area ratio V _α (unit:%) is _expressed by X _{KAM ≦ 0.4 °} / V _α ≧ 0.8
The ratio X _{KAM = 0.6-0.8 °} of the frequency with the KAM value of 0.6-0.8 ° relative to the total frequency is 10-20%, and
Elongation and elongation characterized in that the dispersion state of cementite particles having an equivalent circle diameter of 0.1 μm or more present at the interface between the ferrite and the hard second phase is 3 or less per 1 μm ^{2 of the} hard second phase. A high-strength cold-rolled steel plate with an excellent balance of flangeability. 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. Ingredient composition further
Mo: 0.02-1.0%
Cu: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2, comprising Ni: 0.05-1.0%. Furthermore,
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet excellent in the balance of elongation and stretch flangeability according to any one of claims 1 to 3. A steel material having the composition shown in any one of claims 1 to 4 is hot-rolled under the conditions shown in the following (1) to (4), then cold-rolled, and then annealed. A method for producing a high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability, characterized by tempering.
(1) Hot rolling conditions Finish rolling end temperature: Ar ₃ points or more Winding temperature: 450-700 ° C
(2) Cold rolling conditions Cold rolling rate: 20-80%
(3) Annealing conditions The temperature range of 600 to Ac1 ° C. is raised in a temperature rising pattern that satisfies both the following formulas 1 and 2, and the annealing heating temperature: [(8 × Ac1 + 2 × Ac3) / 10] to 1000 ° C. In the annealing holding time: after holding at 3600 s or less, quench from the annealing heating temperature directly to the temperature below the Ms point at a cooling rate of 50 ° C./s or from the annealing heating temperature to below 600 at the annealing heating temperature. After cooling slowly at a cooling rate (referred to as “first cooling rate”) of 1 ° C./s or more and less than 50 ° C./s until a temperature equal to or higher than 0 ° C. (referred to as “first cooling end temperature”), a temperature below the Ms point (It is referred to as “second cooling end temperature”) and is rapidly cooled at a cooling rate of 50 ° C./s or less (referred to as “second cooling rate”).
(4) Tempering conditions 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 of 5 ° C./s, and [tempering heating temperature−10 ° C.] to tempering heating temperature. Time existing in the temperature region between (referred to as “tempering holding time”): 30 seconds or less, and then cooled at a cooling rate of more than 5 ° C./s.

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