JP4389727B2 - High tensile cold-rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile cold-rolled steel sheet suitable for applications such as automotive pillars and dashboard reinforcing materials, and a method for producing the same, and particularly to improve strength-elongation balance and impact resistance characteristics. Is.

  Conventional high-tensile steel sheets with a tensile strength (TS) of 590 MPa or higher have limited formability due to poor formability. However, in connection with recent exhaust gas regulations due to global environmental problems, the reduction of vehicle weight is a very important issue. On the other hand, ensuring safety is equally important, and it is necessary to improve the impact resistance of the steel sheet. Therefore, the steel plate used as a raw material is required to have high strength and formability and impact resistance that surpass conventional steel plates.

As a high-strength steel sheet for automobiles having excellent collision safety and formability, there is a technique described in Patent Document 1.
This technology has a martensite space ratio of 3 to 30% in the microstructure of the steel sheet, the average grain size of the martensite is 5 μm or less, and the steel sheet has a work hardening index of 0.13 or more and a yield ratio of 75% or less, tensile strength x total elongation is 18000 MPa ·% or more, and hole expansion ratio is 1.2 or more.
In this technology, the impact resistance is evaluated by the work hardening index (n value) in a static tensile test (about 0.001 to 0.01 s ^{-1 in the} JIS standard), but the strength of steel increases with increasing strain rate. The deformation speed of the member at the time of collision reaches 10 to 10 ³ s ⁻¹ , so the evaluation of the static tensile test is insufficient, and the evaluation needs to be performed in consideration of the deformation speed of the member.
When the steel plate manufactured by the prior art as described in Patent Document 1 is evaluated in consideration of the deformation speed of the above-described member, there is a problem that the performance requiring the impact resistance characteristic cannot be satisfied.

Patent Document 2 discloses a technique for producing a ferritic-martensitic steel with improved formability and impact resistance. In this technique, it has been studied to improve the absorbed energy at a strain rate of 2000 s ⁻¹ for impact resistance. The absorbed energy at a strain rate of 2000 s ⁻¹ is a characteristic necessary for absorbing energy at the time of a car collision due to deformation of the member itself. Since the collision energy absorbing member undergoes large deformation in a short time, the deformation strain rate reaches 10 ² to 10 ³ s ⁻¹ . Therefore, it is necessary to improve the absorption energy and static ratio at 10 ² to 10 ³ s ⁻¹ .
Here, the static to dynamic ratio is a high speed (dynamic) with a strain rate of 10 ² to 10 ³ s ^-1 with respect to the strength during static deformation (tensile test) with a strain rate of 10 ^-2 to 10 ^-3 s ^-1. This means the ratio of strength at the time of deformation. The higher this value, the higher the strength at the time of collision and the greater the energy absorption.

However, in order to improve the impact resistance characteristics of the automobile, it is also important to protect the cabin without deforming the parts in order to ensure a living space for the passenger. The member used in such a part has a smaller amount of deformation even at the same collision time than the collision energy absorbing member, so the ultimate strain rate is small, and the absorbed energy at a strain rate of about 10 s ⁻¹ is important. .
Conventionally, however, little means has been studied for improving the absorbed energy at such a strain rate of about 10 s ⁻¹ .

Patent Document 3 discloses a technique in which elongation characteristics are remarkably improved by retaining 10% or more of retained austenite.
However, this technique has not been studied at all for improving the impact resistance.

Patent Document 4 describes a technique related to a steel sheet having excellent impact resistance characteristics in which the martensite fraction and the ratio of the hardness of the martensite phase and the ferrite phase are controlled. This is a technique related to improvement of absorbed energy at a high strain rate of 800 s ⁻¹ , and contributes to improvement of characteristics of the impact energy absorbing member as in the above-described Patent Document 2. Therefore, this technique is different from the technique for improving the absorbed energy at a strain rate of about 10 s ⁻¹ necessary for the member for protecting the cabin aimed by the present invention.
In this technique, the hardness of martensite and ferrite is measured by Vickers hardness measurement, and the value is controlled. However, as clarified in Non-Patent Document 1, the hardness of martensite is dependent on the indentation size, and the result of evaluation with an indentation size of 1 μm or less and the indentation size of 5 μm or more that can be measured with a Vickers hardness tester. The results of the evaluation were different, and in the study by the present inventors, there was no correlation between the impact resistance evaluated by the absorbed energy at a strain rate of about 10 s ^-1 and the hardness measured using a Vickers hardness tester. .

JP 11-61327 A Japanese Patent No. 3253880 JP 61-217529 JP 10-147838 A Proceedings of the International Workshop on the Innovative Structural Materials for Infrastructure in 21st Century (2000), P.185-193, Author: T. Ohmura et al.

The present invention has been developed in view of the above-mentioned present situation, improving the elongation property (El) to improve the strength-elongation balance (TS × El), and the impact resistance property when the deformation strain rate is about 10 s ^−1. An object of the present invention is to propose a high-tensile cold-rolled steel sheet that has been effectively improved along with its advantageous manufacturing method.

Specific characteristic values targeted in the present invention are as follows.
(1) Tensile strength (TS) ≧ 590 MPa
(2) Strength-elongation balance (TS × El) ≧ 16000 MPa ・%
(3) Impact resistance: strain rate is 10s- ¹
(a) Absorbed energy up to 10% strain: 59 MJ · m ⁻³ or more
(b) Absorption energy up to 10% of strain per TS: 1 MPa: 0.100 MJ · m ⁻³ / MPa or more

As a result of intensive studies to achieve the above object, the inventors have obtained the following knowledge.
Obtaining the stress-strain relationship at a strain rate of 10 s ^-1 with high accuracy has been extremely difficult in the past, but it has become possible to measure by using a recently developed test block type impact tensile tester. . Thus, when this apparatus was used to study the improvement of the absorbed energy of the steel sheet at a strain rate of 10 s ⁻¹ , this absorbed energy was a ferritic-martensitic composite structure steel and the dispersion state of the martensite phase and martensite. It has been found that it can be improved by precisely controlling the nanohardness of the phase.
Specifically, the ferrite fraction is 50% or more, the martensite fraction is 10% or more, and the ratio of the phase interval in the rolling direction to the phase interval in the thickness direction of the martensite phase is 0.85 or more and 1.5 or less, It is important that the nanohardness of the martensite phase is 8 GPa or more.

  As an example of a specific method for achieving the dispersion of the martensite structure as described above, adjustment of the winding temperature and the cold rolling reduction ratio and control of the annealing temperature can be mentioned. As the coiling temperature increases, the hot-rolled sheet exhibits a band-like structure, and the steel sheet structure after the subsequent cold rolling-annealing becomes a structure extending in the rolling direction. Moreover, even if the cold rolling reduction is high, the structure extends in the rolling direction, and the martensite phase interval does not become a desired value.

In this regard, the inventors devised a balance of components mainly composed of C, Mn, and Si, controlled the cooling rate after annealing, and increased the coiling temperature during hot rolling and the cold rolling reduction rate as the annealing temperature increased. In addition to generating martensite by setting a high value, the band structure was eliminated as much as possible, and the elongation and impact resistance were successfully improved. Moreover, it succeeded in improving the strength-elongation balance by eliminating the band-like structure. Furthermore, by controlling the nanohardness of the martensite phase, we succeeded in improving the absorbed energy at a strain rate of 10 s ⁻¹ .
As a result, it has become possible to secure higher absorbed energy as compared with conventional steel having the same tensile strength.

In the past, the micro Vickers hardness tester used for measuring the hardness of a minute region is not impossible to measure the hardness of about 5 μm or less, but there is a large variation and there is a problem in accuracy. No correlation was found between the impact resistance evaluated by the absorbed energy at a strain rate of about 10 s ^-1 .
On the other hand, when the hardness of the martensite phase is measured using a nano hardness tester that can measure the hardness of martensite more accurately, it is evaluated for the first time by the absorbed energy at a strain rate of about 10 s ^−1. A correlation was observed between the impact resistance and the nano-hardness of the martensite phase, and as a result, the absorption energy at a strain rate of 10 s ^-1 was successfully improved.

That is, the inventors measured the hardness of martensite with a nano hardness tester capable of measuring the hardness even when the indentation size is 1 μm or less. In this nanohardness test, the measurement was performed with the indentation size substantially the same. Specifically, the hardness was measured by adjusting the load so that the indentation depth (= contact depth) proportional to the size of the indentation was 50 ± 20 nm. At this time, one side of the indentation is about 350 nm. It has been found that when the hardness is 8 GPa or more, a predetermined impact resistance characteristic can be obtained.
In addition, a similar test was attempted using a micro Vickers hardness tester. However, in this test, no correlation was found between hardness, impact resistance and elongation characteristics of the steel of the present invention.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
1. % By mass
C: 0.04% or more, 0.13% or less,
Si: 0.3% or more, 1.2% or less,
Mn: 1.0% or more, 3.5% or less,
P: 0.04% or less,
S: 0.01% or less and
Al: Including 0.07% or less, the balance is Fe and inevitable impurities composition, ferrite fraction is 50% or more, martensite fraction is 10% or more, and the martensite phase relative to the thickness direction A high-tensile cold-rolled steel sheet having a structure in which a ratio of phase intervals in a rolling direction is 0.85 or more and 1.5 or less, and a nano hardness of a martensite phase is 8 GPa or more.

2. In the above 1, the steel plate is further in mass%,
Cr: 0.5% or less,
Mo: 0.3% or less,
A high-tensile cold-rolled steel sheet having a composition containing one or more selected from Ni: 0.5% or less and B: 0.002% or less.

3. In the above 1 or 2, the steel sheet is further in mass%,
Ti: 0.05% or less and
Nb: A high-tensile cold-rolled steel sheet having a composition containing one or two selected from 0.05% or less.

4). The steel slab having the composition according to any one of the above 1 to 3, after hot rolling, 450 ° C. or higher, up winding at 650 ° C. below the temperature, then 30% or more, cold at a reduction ratio of 70% or less After rolling, (winding temperature (° C) + cold rolling reduction (%) × 4.5) (° C) or more, (winding temperature (° C) + cold rolling reduction (%) × 5.5) (° C ) A method for producing a high-tensile cold-rolled steel sheet, which is heated to the following temperature range and then cooled to 340 ° C. or less at an average cooling rate of 10 ° C./s or more.

According to the present invention, it is possible to obtain a high-tensile cold-rolled steel sheet that is excellent in strength-elongation balance and excellent in impact resistance when deformed at a strain rate of about 10 s- ¹ .

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.04% or more, 0.13% or less C is required to contain 0.04% or more in order to appropriately control the tensile strength (TS) and further to make the martensite fraction 10% or more. However, if C exceeds 0.13%, the weldability is remarkably deteriorated, so the upper limit is made 0.13%. Preferably it is 0.07 to 0.12% of range.

Si: 0.3% or more, 1.2% or less
Si is an important element for controlling the dispersion state of martensite and the nano hardness of martensite. In order to prevent softening of martensite during cooling and improve nano hardness, it is necessary to contain 0.3% or more of Si. However, even if it is added in excess of 1.2%, this effect is saturated, and rather the chemical conversion treatment performance is significantly lowered, so the upper limit is made 1.2%. Preferably it is 0.4 to 0.7%.

Mn: 1.0% or more, 3.5% or less
Mn needs to be contained in an amount of 1.0% or more in order to ensure TS ≧ 590 MPa, and can be appropriately contained in an amount of 1.0% or more depending on the required strength. Mn is also extremely effective in improving the nano hardness of martensite. However, excessive addition exceeding 3.5% significantly increases the strength and decreases the elongation, so the upper limit is set to 3.5%. Preferably it is 2.3 to 2.8%.

P: 0.04% or less P is segregated at the prior austenite grain boundaries to deteriorate the low temperature toughness, and because it has a strong tendency to segregate in the steel, the anisotropy of the steel sheet is increased to reduce the workability, so it is reduced as much as possible. However, since it is acceptable up to 0.04%, the P content is limited to 0.04% or less. Preferably it is 0.02% or less.

S: 0.01% or less When S is segregated at the prior austenite grain boundaries or a large amount of MnS is formed, low temperature toughness is lowered and hydrogen-induced cracking is likely to occur. % Is acceptable, so the S content is limited to 0.01% or less. Preferably it is 0.006% or less.

Al: 0.07% or less
Al is added as a steel deoxidizer and is an effective element for improving the cleanliness of the steel, and is preferably contained in an amount of 0.001% or more. However, if the content exceeds 0.07%, a large amount of inclusions are generated, causing defects in the cold-rolled steel sheet, so the upper limit was made 0.07%. More preferably, it is 0.05% or less.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Cr: 0.5% or less
Cr can be used to improve the hardenability and control the amount of martensite. In order to acquire this effect, it is preferable to contain 0.02% or more of Cr. However, if the content exceeds 0.5%, the electrodeposition paintability performed after molding into parts is reduced, so the upper limit was made 0.5%. Preferably it is 0.2% or less.

Mo: 0.3% or less
Mo can be used to improve the hardenability and control the amount of martensite. In order to obtain this effect, it is preferable to contain 0.05% or more, but if it exceeds 0.3%, it will be cooled. Since the hot rolling property is lowered, the content is limited to 0.3% or less. Preferably it is 0.2% or less.

Ni: 0.5% or less
Ni can be used to improve the hardenability and control the amount of martensite. In order to obtain this effect, Ni is preferably contained in an amount of 0.05% or more. Since it causes a decrease in hot rolling property, it is limited to 0.5% or less. Preferably it is 0.3% or less.

B: 0.002% or less B can be used to improve the hardenability and control the amount of martensite. To obtain this effect, B is preferably contained in an amount of 0.0005% or more, but exceeds 0.002%. Therefore, the cold rolling property is lowered, so the content is limited to 0.002% or less. Preferably it is 0.001% or less.

Ti: 0.05% or less
Ti can be used to control the dispersion of the martensite phase by reducing the ferrite grain size. In order to obtain this effect, it is preferable to contain 0.005% or more of Ti. However, if the content exceeds 0.05% and the effect is saturated, the upper limit was made 0.05%. Preferably it is 0.005 to 0.02% of range.

Nb: 0.05% or less
Nb, like Ti, can be used to control the dispersion of the martensite phase by reducing the ferrite grain size. In order to obtain this effect, it is preferable to contain Nb in an amount of 0.005% or more, but even if it is contained in a large amount exceeding 0.05%, the effect is saturated, so the upper limit was made 0.05%. Preferably it is 0.005 to 0.02% of range.

  In the present invention, the balance other than the above-described components is preferably composed of Fe and inevitable impurities. Examples of inevitable impurities include N, O, and Cu. Note that, when N is contained in a large amount, the elongation characteristics may be deteriorated due to aging deterioration, so the content is preferably limited to 0.005% or less.

The preferred component composition range has been described above, but in the present invention, it is not sufficient to limit the component composition to the above range, and as described below, adjustment of the steel structure is also important.
Ferrite fraction: 50% or more In order to achieve a strength-elongation balance (TS x El) of 16000 MPa ·% or more, the ferrite fraction of the entire structure must be 50% or more. That is, when the ferrite fraction is less than 50%, the hard structure other than ferrite increases, so that the strength becomes too high and the strength-elongation balance is lowered. Further, when the strain rate is about 10 s ^{−1, the} amount of increase in stress at the time of deformation of the ferrite portion is large. A preferable range of the ferrite fraction is about 60 to 80%.

Martensite fraction: 10% or more In order to make the strength-elongation balance (TS x El) 16000 MPa ·% or more and to improve the impact resistance, the martensite fraction needs to be 10% or more. That is, when the martensite fraction is less than 10%, sufficient satisfactory impact resistance characteristics cannot be obtained. The preferred range for the martensite fraction is about 20-40%.

  As for the steel structure, in order to sufficiently obtain the effects of the ferrite and martensite described above, the total of the ferrite fraction and the martensite fraction is preferably 90% or more. That is, the other phases other than ferrite and martensite are preferably 10% or less. Here, the other phases other than ferrite and martensite are specifically austenite, bainite, cementite, and pearlite. Needless to say, the smaller the number of these phases, the more preferably austenite is less than 3% because it lowers the impact resistance.

  Note that the ferrite fraction and martensite fraction described above may be determined by observing the structure at the 1/4 thickness position of the thickness cross section parallel to the rolling direction. In other words, the plate thickness section parallel to the rolling direction was mirror-polished and then etched with 1.5% nital, and a 1000x photograph was taken with a scanning electron microscope (SEM) at the plate thickness 1/4 position. And the ferrite portion and other phase portions are measured, the area ratio of each phase is measured, and the measured area ratio of each phase is taken as the fraction of each phase.

Ratio of phase interval in rolling direction to phase interval in thickness direction of martensite phase: 0.85 or more, 1.5 or less Strength-elongation balance (TS x El) is 16000 MPa ·% or more, and strain rate is 10 s ^-1 To achieve an absorption energy of up to 59 MJ · m ⁻³ up to a strain of 10% and an absorption energy of up to 10% strain per TS: 1 MPa, 0.100 MJ · m ⁻³ / MPa The ratio is important. That is, if the phase spacing ratio is less than 0.85 or exceeds 1.5, sufficient satisfactory elongation and impact resistance cannot be obtained.

Since martensite is harder than ferrite and martensite becomes an obstacle to the movement of dislocations, the dislocation (strain) moves preferentially where no martensite is present. Therefore, the larger the ratio of the phase interval in the rolling direction to the phase interval in the plate thickness direction of the martensite phase (that is, the wider the phase interval in the rolling direction compared to the phase interval in the plate thickness direction), or martensite. The smaller the ratio of the phase interval in the rolling direction to the phase interval in the plate thickness direction of the phase (ie, the wider the phase interval in the plate thickness direction compared to the phase interval in the rolling direction), the dislocation (strain) It moves to a longer distance by concentrating on a wide area, that is, a portion that is not martensite.
On the other hand, the ratio of the phase interval in the rolling direction to the phase interval in the plate thickness direction of the martensite phase is close to 1, that is, there is no significant difference between the phase interval in the plate thickness direction of the martensite phase and the phase interval in the rolling direction. Since the movement of dislocation (strain) is suppressed by martensite, the accumulated amount of dislocation (strain) increases, the deformation stress rises, and the impact resistance is improved. Further, since the distribution of the martensite phase becomes relatively uniform, the elongation characteristics are also improved.

  In order to effectively suppress the movement of dislocation (strain) by martensite, it is important to control the ratio of the phase spacing in the rolling direction to the phase spacing in the thickness direction of the martensite phase to 0.85 or more and 1.5 or less. is there. That is, when the ratio of the phase interval in the rolling direction to the phase interval in the plate thickness direction of the martensite phase exceeds 1.5, the phase interval in the rolling direction becomes too wide with respect to the phase interval in the plate thickness direction, but less than 0.85 In addition, since the phase interval in the plate thickness direction becomes too large with respect to the phase interval in the rolling direction, sufficient impact resistance and elongation properties cannot be ensured in any case. In this respect, if the ratio of the phase interval in the rolling direction to the phase interval in the thickness direction of the martensite phase is in the range of 0.85 or more and 1.5 or less, the movement of dislocation (strain) can be effectively suppressed by martensite. . A more preferable range of the phase interval ratio is 1.0 or more and 1.3 or less.

  In the steel sheet of the present invention to be cold-rolled, the phase interval in the plate width direction of the martensite phase is closer to the phase interval in the plate thickness direction than that in the rolling direction, that is, the plate thickness direction of the martensite phase. The ratio of the phase spacing in the sheet width direction to the phase spacing of the steel tends to be closer to 1 than the ratio of the phase spacing in the rolling direction to the phase spacing in the thickness direction of the martensite phase. For this reason, in the present invention, the maximum value of the martensite phase interval in the steel sheet is represented by the rolling direction, and the degree of martensite dispersion is defined by the ratio to the phase interval in the plate thickness direction.

Here, the phase spacing in the thickness direction of the martensite phase, the phase spacing in the rolling direction of the martensite phase, and the ratio of the phase spacing in the rolling direction of the martensite phase to the phase spacing in the thickness direction of the martensite phase are as follows: This is what I asked for.
The phase spacing ratio of the martensite phase is the same as in the case of obtaining the ferrite fraction and martensite fraction described above, and the plate thickness section parallel to the rolling direction is mirror-polished and etched with 1.5% nital to obtain the plate thickness. For the 1/4 position, take a 1000x photo with a scanning electron microscope (SEM), and on this 1000x SEM photo, 50μm straight lines in the direction parallel to the rolling direction and the plate thickness direction are spaced at 20μm or more. 5 was drawn, the interval of the martensite phase existing on the line was measured, the average interval in the rolling direction and the plate thickness direction was determined, and the ratio of the average values was calculated.

FIG. 1 shows the above measurement procedure. In order to avoid complication of the drawing, FIG. 1 will be described by drawing one line in each of the direction parallel to the rolling direction and the thickness direction.
As is apparent from the figure, the average interval between the martensite phases parallel to the rolling direction is expressed by the following formula (a ₁ + a ₂ + a ₃ + a ₄ + a ₅ ) / 5.
On the other hand, the average spacing of the martensite phase in the thickness direction is given by the following formula (b ₁ + b ₂ + b ₃ ) / 3
It is represented by
Therefore, the ratio of the phase interval in the rolling direction to the phase interval in the plate thickness direction of the martensite phase is
{(A ₁ + a ₂ + a ₃ + a ₄ + a ₅ ) / 5} / {(b ₁ + b ₂ + b ₃ ) / 3}
It is represented by

Nanohardness of martensite phase: 8 GPa or more Further, strain rate: Absorption energy up to 10% strain at 10 s ^-1 is 59 MJ · m ^-3 or more, and absorption energy up to 10% strain per TS: 1 MPa In order to obtain an excellent impact resistance property of 0.100 MJ · m ⁻³ / MPa or more, it is important that the nano-hardness of the martensite phase is 8 GPa or more.
When the nano hardness is less than 8 GPa, the impact resistance characteristics are lowered along with the strength-elongation balance (TS × El). The reason for this is considered to be that when the deformation stress of martensite evaluated by nano hardness is low, the effect of suppressing dislocation movement is reduced. The nano-hardness of martensite is better, and the preferred range is 10 GPa or more.

Here, the nano hardness may be measured as follows.
In other words, the nano hardness is measured by grinding the steel plate from the surface to a thickness of 1/4 position, removing grinding distortion by electropolishing, measuring the 15 martensite hardness using TRIBOSCOPE of Hysitron, The average value may be a nano hardness value. The measurement is performed with the indentation size being almost the same. Specifically, the load is adjusted so that the indentation depth (= contact depth) proportional to the indentation size is 50 ± 20 nm, and the hardness is measured. One side of the indentation at this time is about 350 ± 100 nm.

Next, the manufacturing conditions of the present invention will be described.
The molten steel adjusted to the preferred component composition is melted by a generally known method such as a converter, and then cast by a generally known method such as continuous casting to obtain a steel slab.
Next, hot rolling is performed after the heating, and this heat treatment and hot rolling may be performed according to a conventional method and is not particularly limited. However, the processes after hot rolling need to be performed under the following conditions.

Winding temperature: 450 ° C or more, 650 ° C or less If the winding temperature after hot rolling is less than 450 ° C, the strength of the hot-rolled sheet increases and cold rollability deteriorates, and breaks during continuous cold rolling. The risk of doing is increased. On the other hand, when the temperature exceeds 650 ° C., the development of the band-like structure becomes remarkable, and even if the cold rolling reduction ratio and the annealing temperature are controlled, the band-like structure cannot be eliminated, and the phase spacing ratio of the martensite phase is controlled within a predetermined range. Can not do it. A preferable winding temperature is 500 ° C. or more and 650 ° C. or less.

Cold rolling reduction ratio (cold rolling reduction ratio): 30% or more and 70% or less If the cold rolling reduction ratio is less than 30%, in addition to forming a coarse structure, the martensite phase interval in the plate thickness direction The ratio of the phase spacing in the rolling direction with respect to is less than 0.85, the target phase spacing ratio of the martensite phase cannot be obtained, and the elongation and impact resistance properties deteriorate. On the other hand, if it exceeds 70%, the band-like structure cannot be resolved even if the hot rolling coiling temperature and annealing temperature are controlled, and the ratio of the phase spacing in the rolling direction to the phase spacing in the thickness direction of the martensite phase exceeds 1.5. Also, the phase interval ratio of the martensite phase cannot be set within a predetermined range.

Heating temperature range: (rolling temperature (° C) + cold rolling reduction (%) x 4.5) (° C) or more, (winding temperature (° C) + cold rolling reduction (%) × 5.5) (° C) or less The higher the rolling temperature and the cold rolling reduction rate, the more easily a band-like structure is formed. In order to eliminate this, it is necessary to heat at a high temperature after cold rolling.
If the heating temperature is lower than the above temperature range, the band-like structure cannot be resolved, the target phase spacing ratio of the martensite phase cannot be obtained, and the diffusion of substitutional atoms of Si and Mn is insufficient. The nano hardness of the martensite phase cannot be 8 GPa or more. On the other hand, if the heating temperature exceeds the above temperature range, the austenite at the time of heating is unevenly dispersed, so the target martensite phase interval ratio cannot be obtained, and the austenite grains are probably coarsened, Since the martensite block size becomes coarse, the nano-hardness of martensite cannot be increased to 8 GPa or more, and the elongation and impact resistance properties deteriorate.
In order to control the ratio of the phase interval in the rolling direction to the phase interval in the thickness direction of the martensite phase to a more preferable range, it is more preferable to heat to an austenite single phase region exceeding Ac ₃ in the above range. In particular, when the cold rolling reduction is as high as 60% or more, it is particularly preferable to heat to the austenite single phase region.

  Also, if the holding time in this heating temperature range is short, the martensite fraction has a large dependency on the holding time, so it may be difficult to secure 10% or more of martensite depending on the conditions, and the total coil length Since it may be difficult to obtain a stable material over a long period of time, the holding time in this heating temperature region is preferably 30 seconds or more. On the other hand, if the holding time exceeds 600 seconds, the effect is saturated and only the manufacturing cost is increased. Therefore, the upper limit of the holding time is preferably 600 seconds.

  In addition, after heating to the said temperature range, it is necessary to quench at a speed | rate of 10 degrees C / s or more, and in order to set it as such a heat history, it is advantageous to process in a continuous annealing furnace. In this case, in order to hold for 30 seconds or more in the above temperature range, the annealing temperature at the time of continuous annealing (the highest temperature reached in the continuous annealing furnace) is set as the temperature of the temperature range, and the holding time in the temperature range is 30 seconds or more. For example, the soaking time at the annealing temperature (also referred to as annealing time) may be 30 seconds or more, and after reaching the annealing temperature, gradually cooled to the lower limit of the temperature range, The holding time in the temperature range may be 30 seconds or longer.

Average cooling rate: Cooling to 340 ° C or less at a rate of 10 ° C / s or higher If the cooling rate is less than 10 ° C / s or the cooling stop temperature is higher than 340 ° C, the target nano hardness of martensite cannot be obtained. Here, the cooling rate is an average cooling rate from the lower limit temperature of the heating temperature range, that is, (winding temperature (° C.) + Cold rolling reduction ratio (%) × 4.5) (° C.) to the controlled cooling stop temperature.
On the other hand, at a cooling rate exceeding 50 ° C./s, the cooling tends to be non-uniform and predetermined characteristics may not be obtained over the entire plate width. Therefore, the cooling rate is preferably 50 ° C./s or less. Further, such controlled cooling is preferably performed to 300 ° C. or lower, more preferably to 275 ° C. or lower. In addition, after this controlled cooling, it is not necessary to specify in particular, For example, it may be air cooling (cooling), may be slow cooling, and should just cool to about room temperature by a well-known method. However, it is preferable to avoid re-heating and tempering because it reduces the nanohardness of martensite.

Steel with the component composition shown in Table 1 is melted with a converter and RH degassing equipment and made into a slab by continuous casting. The slab is heated to 1100-1250 ° C and then wound under the conditions shown in Table 2. Then, cold rolling treatment, continuous annealing treatment, and controlled cooling treatment were similarly performed under the conditions shown in Table 2 to obtain a high-tensile cold-rolled steel sheet.
In addition to observing the steel structure of the high-tensile cold-rolled steel sheet thus obtained, a normal static tensile test, a high-speed tensile test at a strain rate of 10 s ⁻¹ and a nanohardness test using a test force block method were performed.
The obtained results are shown in Table 3.
Incidentally, Ac ₃ shown in Table 1, for samples cut out from the sheet bar taken after rough rolling, using a Fuji Telecommunications Koki Co. Sir Mech master Z, experimentally determined.

The steel structure and each characteristic were measured as follows.
(a) Measurement of the ferrite fraction and martensite fraction by structural observation and the measurement of the phase spacing ratio of the martensite phase were performed at the position of the thickness 1/4 of the thickness cross section parallel to the rolling direction.
Ferrite and martensite fractions were mirror-polished on the plate thickness, etched with 1.5% nital, and photographed 1000 times with a scanning electron microscope (SEM) at the plate thickness 1/4 position. The part, the ferrite part, and other phase parts were identified, and the area ratio of each phase was quantitatively evaluated by image analysis to obtain the fraction of each phase. In addition, the types of other phase portions were determined by observation with the SEM.
Here, for No. 37 in Table 3, since the type of tissue could not be determined only by the observation by SEM of the above-mentioned other phase portions, a sample subjected to SEM observation was used, and EBSP (Electron Back Analysis by Scatter diffraction pattern) confirmed that the other phases were austenite.
The martensite phase spacing ratio was determined by drawing five 50μm straight lines at intervals of 20μm or more in the direction parallel to the rolling direction and the plate thickness direction on the 1000x SEM photograph taken as described above. The distance between the martensite phases present in the film was measured, the average distance in the rolling direction and the sheet thickness direction was determined, and the ratio of the average values was calculated.

FIG. 1 shows the above measurement procedure. In order to avoid complication of the drawing, FIG. 1 will be described by drawing one line in each of the direction parallel to the rolling direction and the thickness direction.
As is apparent from the figure, the average interval between the martensite phases parallel to the rolling direction is expressed by the following formula (a ₁ + a ₂ + a ₃ + a ₄ + a ₅ ) / 5.
On the other hand, the average spacing of the martensite phase in the thickness direction is given by the following formula (b ₁ + b ₂ + b ₃ ) / 3
It is represented by
Therefore, the ratio of the phase interval in the rolling direction to the phase interval in the plate thickness direction of the martensite phase is
{(A ₁ + a ₂ + a ₃ + a ₄ + a ₅ ) / 5} / {(b ₁ + b ₂ + b ₃ ) / 3}
It is represented by

(b) The static tensile properties were measured by a method based on JIS Z 2241 using a JIS No. 5 test piece having a longitudinal direction perpendicular to the rolling direction.
(c) A high-speed tensile test at a strain rate of 10 / s using the test force block method is performed using a test force block type impact tensile tester (TS-2000) manufactured by Sasamiya Seisakusho, in a direction perpendicular to the rolling direction. : The test was performed under the condition of 10 s ⁻¹ .
(d) The nano-hardness is measured from the surface to 1 / 4th of the plate thickness, and after grinding strain is removed by electrolytic polishing, the hardness of martensite is measured at 15 points using TRIBOSCOPE from Hysitron, and the average is obtained. The value was taken as the hardness value. The measurement was performed with the indentation size being almost the same. Specifically, the load was adjusted so that the indentation depth (= contact depth) proportional to the size of the indentation was 50 ± 20 nm, and the hardness was measured. At this time, one side of the indentation is about 350 ± 100 nm.

As shown in Table 3, all the inventive examples not only have an excellent strength-elongation balance of (TS × El) ≧ 16000 MPa ·%, but also up to 10% strain at a strain rate of 10 s ⁻¹ . It has excellent impact resistance with an absorbed energy of 59 MJ · m ⁻³ or more and an absorbed energy up to 10% strain per TS: 1 MPa of 0.100 MJ · m ⁻³ / MPa or more.

  The high-tensile cold-rolled steel sheet having excellent strength-elongation balance and impact resistance characteristics of the present invention can reduce the plate thickness of automobile parts and improve the collision safety of automobiles, and greatly contributes to the enhancement of the performance of automobile bodies.

It is the figure which showed the measuring point of ratio of the phase spacing of the rolling direction with respect to the phase spacing of the thickness direction of a martensite phase.

Claims (4)

% By mass
C: 0.04% or more, 0.13% or less,
Si: 0.3% or more, 1.2% or less,
Mn: 1.0% or more, 3.5% or less,
P: 0.04% or less,
S: 0.01% or less and
Al: Including 0.07% or less, the balance is Fe and inevitable impurities composition, ferrite fraction is 50% or more, martensite fraction is 10% or more, and the martensite phase relative to the thickness direction A high-tensile cold-rolled steel sheet having a structure in which a ratio of phase intervals in a rolling direction is 0.85 or more and 1.5 or less, and a nano hardness of a martensite phase is 8 GPa or more. In claim 1, the steel sheet is further in mass%,
Cr: 0.5% or less,
Mo: 0.3% or less,
A high-tensile cold-rolled steel sheet having a composition containing one or more selected from Ni: 0.5% or less and B: 0.002% or less. In Claim 1 or 2, a steel plate is further mass%,
Ti: 0.05% or less and
Nb: A high-tensile cold-rolled steel sheet having a composition containing one or two selected from 0.05% or less. A steel slab having the composition according to any one of claims 1 to 3 is wound after hot rolling at a temperature of 450 ° C or higher and 650 ° C or lower, and then at a rolling reduction of 30% or higher and 70% or lower. After cold rolling, (winding temperature (° C.) + Cold rolling reduction rate (%) × 4.5) (° C.) or more, (winding temperature (° C.) + Cold rolling reduction rate (%) × 5.5) ( A method for producing a high-tensile cold-rolled steel sheet, wherein the steel sheet is heated to a temperature range of:

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