JP2013227624A - Method of manufacturing high strength cold rolled steel sheet excellent in workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet excellent in workability, especially shape fixability at press working and total elongation.SOLUTION: A steel slab has a composition that includes, by mass, C:0.05-0.12%, Si:at most 0.5%, Mn:2.0-4.0%, Ti:0.005-0.06%, Nb:0.005-0.08%, Al:at most 0.1%, and a remainder consisting of Fe and inevitable impurities, wherein the steel slab is used as a raw material. In a series of manufacturing processes, especially in a continuous annealing, an average temperature rise speed V, within a temperature range from 700°C to a steel sheet temperature T(°C) provided by the following formula (1) in a course of a temperature increase, is 0.3-8°C/s. The formula (1) is represented: T=0.98T, wherein Tis a highest attainment temperature (°C) of the steel sheet in the continuous annealing, and is in a range of at least Acpoint and less than Acpoint.

Description

  The present invention is mainly used for seat parts such as a seat frame and automobile parts such as bumpers and impact beams, and is a method for producing a high-strength cold-rolled steel sheet excellent in workability, in particular, shape freezing property during press working and total elongation. It is about.

  In order to improve the fuel economy of automobiles and ensure collision safety by reducing the weight, the need for thin steel sheets with higher strength than ever is increasing, and the use frequency of steel sheets with a tensile strength of 780 MPa or more, or even 980 MPa or more. Is growing. Such high-strength steel plates are used as automobile frame parts and reinforcing parts, and these parts are generally manufactured by pressing.

  From the viewpoint of preventing cracking during pressing, high strength steel sheets are required to have high total elongation. Specifically, a total elongation El of 22% or more, more preferably 24% or more, and a steel sheet of 980 MPa class strength of 15% or more, more preferably 17% or more is desired for a steel plate having a strength of 780 MPa.

  At the same time, a low yield ratio high strength cold-rolled steel sheet with low yield strength is often desired from the viewpoint of preventing shape defects due to springback after processing, that is, the requirement for shape freezing. That is, the steel sheet has a low yield ratio, which is the ratio of the yield strength to the tensile strength. However, the lower the yield strength or the yield ratio, the better. It is necessary to ensure a yield strength of a certain value or more in order to prevent plastic deformation of the part.

  Therefore, a low yield ratio, high strength cold-rolled steel sheet requires a certain level of tensile strength and a certain range of yield strength. According to the Japan Iron and Steel Federation standard, 980 MPa class (sheet thickness 1.0-3.2 mm) cold-rolled For steel sheets, the yield strength is defined as YS: 590 to 930 MPa (yield ratio YR: 0.60 to 0.95). In recent years, a steel sheet having a yield ratio of 0.90 or less, or even 0.85 or less is required from the viewpoint of securing better shape freezing property.

  As such a low yield ratio high strength cold rolled steel sheet, a DP steel sheet composed of two phases of ferrite and martensite is suitable. DP steel sheet achieves high strength and low yield ratio by dispersing hard martensite phase in soft ferrite phase (hereinafter also referred to as α phase). However, in addition to high strength and low yield ratio, there is a problem that if high total elongation is simultaneously achieved, the additive amount of elements such as C and Mn is increased and weldability is lowered.

It is also known that workability such as total elongation El is improved by increasing the ratio of Si in the steel, and many high-strength steel sheets have improved workability by adding Si. However, when Si in steel increases, it becomes easy to form stable SiO _{2 on the} surface of the steel sheet, and chemical conversion treatment properties and plating properties deteriorate. In order to prevent these, a technique for performing a special treatment by continuous annealing has been proposed, but there have been practical problems such as an increase in equipment cost.

In addition, for example, Patent Document 1 includes adding predetermined P, and defining the residence time in the temperature range of 950 ° C. from the Ac ₁ transformation point (hereinafter simply referred to as Ac ₁ point) and the subsequent cooling rate. A technique for manufacturing a low-yield-ratio high-tensile steel sheet with good ductility and secondary work brittleness resistance is disclosed. However, with this technique, there is a problem that, if an attempt is made to obtain a high tensile strength of 780 MPa or more, the amount of P added to the steel must be increased, and sufficient chemical conversion treatment properties cannot be obtained.

  Patent Document 2 uses a steel slab containing a predetermined component including Ti, controls the ratio of Ti and S, and performs a predetermined annealing rate when performing continuous annealing held in a two-phase region of ferrite-austenite. The manufacturing technique by cooling in is disclosed. However, this technique has a problem that it is inferior in chemical conversion treatment because a high Si addition amount is required to produce a high-strength steel sheet.

  Patent Document 3 discloses a steel sheet having a texture within an appropriate range in a composite structure steel sheet in order to achieve both workability and shape freezing property. However, in the examples, the total elongation El is not shown, and it is not necessarily considered that desired elongation characteristics can be obtained.

  Patent Document 4 discloses a low yield ratio high strength cold-rolled steel sheet for controlling the texture and the r value, and a manufacturing technique thereof. However, this technique has a problem that the chemical conversion processability is also inferior because a high Si content is required to achieve high strength and high total elongation.

JP 58-22332 A JP 2002-69574 A JP 2004-124123 A JP 2005-256020 JP

  The present invention advantageously solves the above problems, and is excellent in workability, in particular, shape freezing and total elongation during press working, and also in high-strength cold-rolled steel sheets that are excellent in chemical conversion treatment and weldability. The object is to provide an advantageous production method.

As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
(a) From the viewpoint of weldability and chemical conversion property, in the ferrite-martensite dual phase steel in which the addition amount of C and Si is reduced, the addition amount of Mn is increased and simultaneously Ti and Nb are added. Sufficient strength can be obtained.
This is because the Ac ₁ point is lowered by reducing the addition amount of Si and increasing the addition amount of Mn, and the recrystallization temperature is increased by adding Ti and Nb. This is probably because the / γ transformation occurs. Moreover, it is thought that the strength improvement by precipitation strengthening is also achieved by adding Ti and Nb.
(b) For steels with the above-mentioned composition, the temperature rise rate, cooling rate and their relationship in a specific temperature range during continuous annealing are optimally controlled, and the maximum temperature reached by the steel sheet during continuous annealing is at least Ac ₁ point or more By setting the range to less than Ac ₃ points, a high total elongation and a low yield ratio can be realized simultaneously.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.05 to 0.12%,
Si: 0.5% or less,
Mn: 2.0-4.0%
Ti: 0.005-0.06%,
Nb: 0.005-0.08% and
Al: A steel slab containing 0.1% or less, the balance being Fe and inevitable impurities, is hot-rolled, pickled, cold-rolled, and then subjected to temper rolling after continuous annealing. In the method of manufacturing a cold-rolled steel sheet comprising:
In the temperature increase process of the continuous annealing, the average temperature increase rate V ₁ in the temperature range from 700 ° C. to the steel plate temperature T ₁ (° C.) defined by the following formula (1) is 0.3 to 8 ° C./s,
A method for producing a high-strength cold-rolled steel sheet excellent in workability, characterized in that the maximum temperature T _M in the continuous annealing is in the range of Ac ₁ point or more and less than Ac ₃ point.
T ₁ = 0.98T _M (1)
However, the T _M is the maximum temperature of the steel sheet in the continuous annealing (° C.).

2. 2. The workability as described in 1 above, wherein in the temperature raising process of the continuous annealing, an average temperature rising rate V ₀ in a temperature range from 300 ° C. to at least 650 ° C. is set in a range defined by the following formula (2). For producing high-strength cold-rolled steel sheets with excellent resistance.
Record
2.0 ・ V ₁ ≦ V ₀ ≦ 20 ・ V ₁ (2)

3. In the cooling process of the continuous annealing, the average cooling rate V ₂ in the temperature range from the maximum temperature T _M to the steel plate temperature T _{1 is set} to a range represented by the following formula (3),
_{3. The} method for producing a high-strength cold-rolled steel sheet having excellent workability according to 1 or 2 above, wherein an average cooling rate V3 in a temperature range from 700 ° C to at least 400 ° C is 10 to 80 ° C / s.
Record
{0.5 / (V ₁ +0.3)} + 0.3 ≦ V ₂ ≦ {3 / (V ₁ +1)} + 0.7 (3)

4). The steel slab is further mass%,
B: 0.0005 to 0.0030%,
Mo: 0.05-2%
V: 0.05-0.5% and
Cr: 0.01-1%
The method for producing a high-strength cold-rolled steel sheet having excellent workability as described in any one of 1 to 3 above, which comprises one or more selected from among the above.

According to the present invention, it is possible to produce a high-strength cold-rolled steel sheet that is excellent in workability, particularly shape freezing property and total elongation at the time of press working, and further excellent in chemical conversion property and weldability.
The high-strength cold-rolled steel sheet produced according to the present invention is suitable mainly as a material for seat parts such as a seat frame and automobile parts such as bumpers and impact beams.

V ₁ and is a diagram showing the relationship between the total elongation El. V ₁ and V ₀ is a diagram showing the effect on total elongation El. V ₁ and V ₂ is a diagram showing the effect on yield ratio YR and total elongation El.

The mechanical characteristics targeted by the present invention are as follows.
Tensile strength TS: 780MPa or more Yield strength YS: TS 780MPa class 470-740MPa
TS 980MPa class 590 ~ 930MPa
Here, the yield strength YS is 0.2% proof stress.
Yield ratio YR: 0.60-0.90, preferably 0.60-0.85
Total elongation El: TS 780 MPa class 22% or more, preferably 24% or more
TS 980MPa class 15% or more, preferably 17% or more

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition is limited to the above range will be described. The unit of the content of each element is mass% unless otherwise specified.
C: 0.05-0.12%
C is an additive element that is essential for strengthening the structure by forming martensite and retained austenite in the steel. It is possible to improve both strength and elongation by increasing the amount of C added, but such an effect does not appear unless the amount added is less than 0.05%. On the other hand, if the addition amount is large, the welded portion becomes brittle and the welding strength is lowered. Therefore, the C content is in the range of 0.05 to 0.12%.

Si: 0.5% or less
The strength and elongation can be improved by adding Si. In ferrite-martensite two-phase steel, preferably by adding 0.1% or more of Si, the ferrite phase is strengthened by solid solution strengthening, and the C concentration in the austenite phase (hereinafter also referred to as γ phase) is increased. And, since the martensite phase is strengthened and becomes high strength, a low yield ratio is easily obtained. However, when the amount of Si increases, the amount of SiO ₂ formed on the steel sheet surface increases and the chemical conversion treatment performance deteriorates. If the Si content is 0.5% or less, such adverse effects are small, and a product with good chemical conversion treatment properties can be obtained without using special equipment with high equipment costs. Further, when the Si amount is increased, the Ac ₁ point is increased and recrystallization during continuous annealing is completed in the α single phase region, so that the obtained strength is decreased. From the above viewpoint, the Si content is 0.5% or less. Since the strength tends to decrease when the Si content is less than 0.1%, the Si content is preferably 0.1% or more.

Mn: 2.0-4.0%
Mn is an element that increases the hardenability of the steel, and has the effect of increasing the strength by increasing the amount of martensite produced according to the amount added. In addition, Ac ₁ point decreases with increasing Mn content, and transformation to γ phase starts before recrystallization is completed, so it contributes to strength improvement by using it simultaneously with Si content reduction. Here, in order to ensure a desired strength, the amount of Mn is set to 2.0% or more. On the other hand, if added over 4.0%, the amount of ferrite phase becomes too small and not only the total elongation El decreases, but also the brittleness of the welded part is caused, so the Mn content is made 4.0% or less.

Ti: 0.005-0.06%, Nb: 0.005-0.08%
Ti forms precipitates such as TiC, TiN or Ti (C, N) in the steel at about 1000 ° C. or less, and Nb forms NbC, NbN or Nb (C, N in the steel at about 1000 ° C. or less. ) And the like, and contributes to precipitation strengthening. Furthermore, the addition of Ti and Nb has the effect of increasing the recrystallization start temperature, thereby contributing to the improvement of strength. However, such precipitation strengthening has a large contribution to the strengthening of the ferrite phase in the ferrite-martensite two-phase structure. Therefore, an excessive amount of Ti and Nb precipitates causes an increase in yield ratio, and further increases elongation. It causes deterioration of the hole expandability. Therefore, it is important to properly control the addition amounts of Ti and Nb.

  In the present invention, by adding Ti and Nb with different precipitation temperatures and different precipitation forms, workability can be ensured and strength can be increased at the same time. It is necessary to set the range of Ti: 0.005 to 0.06% and Nb: 0.005 to 0.08%. If the content is below these, the required strength cannot be obtained, while if it exceeds these, the yield ratio and the total elongation are adversely affected.

Al: 0.1% or less
Al is added for deoxidation of steel and has the effect of strengthening ferrite. However, if added over 0.1%, coarse inclusions increase and workability deteriorates, so the content is made 0.1% or less.

  In the present invention, in addition to the components described above, B, Mo, V and Cr can be added as appropriate. B, Mo, V and Cr are all effective components for improving the tensile strength. From this viewpoint, B, Mo, V and Cr are B: 0.0005 to 0.0030% and Mo: 0.05 to 2%, respectively. V: 0.05 to 0.5%, Cr: 0.01 to 1% are preferable.

  Further, as an impurity, S exists as a non-metallic inclusion in steel and becomes a stress concentration source at the time of stretch flange molding, so it is desirable to reduce its content as much as possible. However, if the amount of S is 0.005% or less, the hole expandability is not adversely affected. Therefore, 0.005% is allowable as the upper limit. More preferably, it is 0.002% or less.

  P not only causes unevenness of the structure, but also causes solidification segregation during casting, leading to internal cracks and deterioration of workability. From such a viewpoint, the P content is preferably 0.05% or less.

  N is easy to bond with Ti and is fixed in steel as TiN and detoxified. However, if it is excessively contained, precipitates are formed at high temperatures, and it becomes coarse precipitates in steel and deteriorates workability. , 0.0050% or less is preferable.

Next, the manufacturing conditions of the present invention will be described.
In the present invention, the steel slab adjusted to the above component composition is heated to a predetermined temperature, hot-rolled, wound into a coil, and after being subjected to a treatment of holding at the winding temperature as necessary, After pickling to remove oxides on the surface, it is cold-rolled to the product thickness. In addition, what is necessary is just to perform in accordance with a conventional method about hot rolling, pickling, and cold rolling.

  The cold-rolled coil thus obtained is continuously annealed. The role of this continuous annealing is high by recrystallizing the cold-rolled structure to release excessive dislocations and making the structure uniform, and in the cooling process, the martensite phase appears in the steel by quenching by quenching. It is about achieving strength. In the present invention, high total elongation can be achieved simultaneously in addition to high strength and low yield ratio by combining the component composition as described above and the conditions of continuous annealing shown below.

Maximum attainment temperature T _M in continuous annealing: Ac ₁ point or more and less than Ac ₃ point In continuous annealing, the maximum attainment temperature T _M of the steel sheet during annealing is set in the range of Ac ₁ point or more and less than Ac ₃ point of the material. This is because a predetermined amount of γ phase appears before entering the cooling process, and by quenching from this state, a ferrite-martensite two-phase structure is obtained after annealing. In the case of a ferrite-martensite two-phase structure (DP structure), high strength is ensured in the martensite phase, and high total elongation is simultaneously achieved by the deformation of the ferrite phase. If the maximum temperature T _M falls below the Ac ₁ point, the γ phase is insufficient and sufficient strength cannot be obtained, while if it exceeds the Ac ₃ point, the α phase becomes insufficient and the total elongation El decreases.

Average heating rate V _{1 in} the temperature range from 700 ° C. to steel plate temperature T ₁ (° C.): 0.3 to 8 ° C./s
Here, the steel plate temperature T ₁ is expressed by the following equation (1).
T ₁ = 0.98T _M (1)
It is the temperature represented by.
Here, the steel plate temperature T ₁ is defined as described above for the following reason.
That is, in the temperature rising process at 700 ° C. or higher, it is necessary to promote the recrystallization of the α phase and optimize the dispersion of the soft α phase. On the other hand, as the temperature approaches T _M , the tendency of the transformation from the α phase to the γ phase proceeds. For this reason, in order to promote recrystallization of the α phase, it is necessary to maintain a low temperature increase rate between 700 ° C. and a temperature slightly lower than T _M. Therefore, T ₁ is defined as described above as the upper limit of the controlled heating temperature range.

In the present invention, the Ac ₁ point is lowered by suppressing the Si addition amount and increasing the Mn addition amount, and the dislocation in the steel is sufficiently achieved by lowering the recovery / recrystallization start temperature by adding Ti or Nb. The α / γ transformation is caused in a high state to increase the γ phase ratio during holding at a high temperature, and a high martensite phase ratio is realized after rapid cooling.
In continuous annealing, which requires high productivity, it is desirable to finish annealing in a short time. For this purpose, it is effective to increase the heating rate. However, when steel with limited amounts of addition of C and Si as in the present invention is strengthened with Ti or Nb, the total elongation El is improved by setting the rate of temperature rise to be lower than a general value. Along with this, it was found that the strength also increased.

In FIG. 1, C: 0.08%, Si: 0.3%, Mn: 2.5%, P: 0.01%, S: 0.003%, Al: 0.03%, N: 0.0030%, Ti: 0.02%, Nb: 0.03%, balance For steel (TS: 780 MPa class) with a component composition of Fe, V ₁ was varied in the range of 0.1 to 10 ° C./s under the conditions of T _M : 780 ° C., and therefore T ₁ : 764.4 ° C. The result of having investigated about total elongation El of a rolled steel sheet is shown. In addition, the average temperature increase rate V _{0 in} the temperature range from 300 ° C. to at least 650 ° C .: 3 ° C./s, the average cooling rate V _{2 in} the temperature range from the maximum temperature T _M to the steel plate temperature T ₁ : 1.5 ° C. / s, average cooling rate V ₃ in the temperature range from 700 ° C. to at least 400 ° C .: 15 ° C./s.
From the figure, it is understood that when V ₁ is 0.3 to 8 ° C./s, the total elongation El is 25% or more, and a stable and high total elongation can be obtained. On the other hand, when V ₁ deviates from 0.3 to 8 ° C./s, it can be seen that the total elongation El is significantly reduced.
Based on the above results, the average rate of temperature increase V ₁ in the temperature range from 700 ° C., which is a relatively high temperature range, to the steel plate temperature T ₁ (° C.) determined by the above formula (1) is slower than in the past. Specifically, it has been determined that by setting 0.3 to 8 ° C./s, a high total elongation can be obtained simultaneously with a high strength and a low yield ratio.

The reason for this is not always clear, but the inventors consider as follows.
That is, in the steel having the component composition of the present invention, since the transformation start temperature is low, the transformation proceeds in a state where there are sufficient lattice defects such as dislocations, which are γ production sites, during continuous annealing. After a certain amount of such γ phase is secured, it is necessary to properly disperse the soft α phase. However, if the temperature rise rate is too high in the high temperature range, recrystallization of the untransformed α phase is difficult to occur. As a result, the residual amount of α phase is insufficient, the soft phase is insufficient after cooling, and the elongation is lowered. For this reason, the average rate of temperature increase V ₁ in the temperature range from 700 ° C., which is a relatively high temperature range, to the steel plate temperature T ₁ (° C.) is controlled slower than the prior art, specifically 0.3-8 ° C./s. We believe that high overall growth can be obtained.

Average heating rate V _{0 in} the temperature range from 300 ° C. to at least 650 ° C .:
2.0 ・ V ₁ ≦ V ₀ ≦ 20 ・ V ₁
In addition, the effect of the heating rate up to 700 ° C on the total elongation El was also examined. As a result, a further improvement in the total elongation El was observed by raising the temperature range from 300 ° C. to at least 650 ° C. at an appropriate rate of temperature rise.

In FIG. 2, C: 0.09%, Si: 0.5%, Mn: 3.2%, P: 0.02%, S: 0.002%, Al: 0.05%, N: 0.0030%, Ti: 0.03%, Nb: 0.08%, balance For steel (TS: 980 MPa class) with a component composition of Fe, the total elongation El of the cold-rolled steel sheet produced by varying V ₀ and V ₁ under the conditions of T _M : 800 ° C. and therefore T ₁ : 784 ° C. The result of having investigated about is shown. The average cooling rate V _{2 in} the temperature range from the highest temperature T _M to the steel plate temperature T _{1 is} 1 ° C./s, and the average cooling rate V ₃ in the temperature range from 700 ° C. to at least 400 ° C. is 25 ° C./s. It was.
From the figure, it can be seen that when V ₁ is 0.3 to 8 ° C./s, the total elongation of 15% or more is obtained in any case. In addition to V ₁ , V ₀ is _expressed by the following formula (2)
2.0 ・ V ₁ ≦ V ₀ ≦ 20 ・ V ₁ (2)
When the above range is satisfied, it can be seen that the total elongation is 17% or more.
From the above, it has been found that the total elongation El can be further improved by controlling V ₀ within the range defined by the above equation (2) in addition to the above-described control of V ₁ .

Although this reason is not necessarily clear, the inventors think as follows.
That is, since 300 to 650 ° C. corresponds to a recovery temperature range, it is necessary to set an appropriate recovery amount in this recovery temperature range in order to obtain a uniform structure after transformation and recrystallization in the subsequent high temperature range, The appropriate amount of recovery is determined by the balance between V ₀ and V ₁ .
For this reason, when V ₀ is less than 2.0 · V ₁ , recovery in the low temperature region proceeds excessively, and recrystallization proceeds partially to make the structure non-uniform, while when V ₀ exceeds 20 · V ₁ Since transformation and recrystallization in the high temperature region proceed with dislocations more than the ideal amount, the structure becomes non-uniform in this case as well, and as a result, the total elongation El is considered to decrease.

  In addition, there is no restriction | limiting in particular about the temperature range from 650 degreeC to 700 degreeC, You may heat with the temperature increase rate as it is, or may change it.

Average cooling rate V _{2 in} the temperature range from maximum temperature T _M to steel plate temperature T ₁ :
{0.5 / (V ₁ +0.3)} + 0.3 ≤ V ₂ ≤ {3 / (V ₁ +1)} + 0.7
Next, the effect of the cooling rate from the maximum temperature T _M on the total elongation El was also examined. As a result, it was found that the yield ratio can be further reduced in addition to the improvement of the total elongation El by cooling the temperature range from the maximum temperature T _M to the steel plate temperature T ₁ at an appropriate cooling rate.

FIG. 3 shows C: 0.08%, Si: 0.3%, Mn: 2.9%, P: 0.01%, S: 0.003%, Al: 0.03%, N: 0.0030%, Ti: 0.02%, Nb: 0.03%, Cr: A cold-rolled steel sheet manufactured by varying V ₁ and V ₂ under the conditions of T _M : 780 ° C. and therefore T ₁ : 764 ° C. The result of having investigated about total elongation El and yield ratio YR of this is shown. The average temperature increase rate V _{0 in} the temperature range from 300 ° C. to at least 650 ° C. was 10 ° C./s, and the average cooling rate V ₃ in the temperature range from 700 ° C. to at least 400 ° C. was 20 ° C./s.
From the figure, it can be seen that when V ₁ is 0.3 to 8 ° C./s, a total elongation of 15% or more and a yield ratio of 0.60 to 0.90 can be obtained. In addition to V ₁ , V ₂ is expressed by the following formula (3)
{0.5 / (V ₁ +0.3)} + 0.3 ≦ V ₂ ≦ {3 / (V ₁ +1)} + 0.7 (3)
When the above range is satisfied, a total elongation of 17% or more and a yield ratio of 0.60 to 0.85 are obtained. By controlling V ₂ within a predetermined range in addition to V ₁ , a higher total elongation can be obtained. It can be seen that a low yield ratio can be achieved simultaneously.
From the above, in addition to the above-described control of V ₀ and V ₁ , it has been found that by controlling V ₂ within the range defined in the above equation (3), improvement in total elongation El and further reduction in yield ratio can be achieved simultaneously. did.

The reason for this is not always clear, but the inventors consider as follows.
That is, in the cooling process in the temperature range from T _M to T ₁ , the appearance of the α phase from the γ phase and the growth of the untransformed α phase occur. Moreover, the growth of α phase is also promoted by slowing V ₁ described above. For this reason, if V ₂ falls below {0.5 / (V ₁ +0.3)} + 0.3, the amount of α phase becomes excessive and the strength is insufficient, while V ₂ becomes {3 / (V ₁ +1)}. If it exceeds +0.7, the amount of α phase becomes insufficient, and the total elongation El decreases. Therefore, by setting the appropriate range in which both effects do not become excessive, the total elongation El can be improved and the yield ratio can be further reduced.

Average cooling rate V ₃ in the temperature range from 700 ° C. to at least 400 ° C .: 10 to 80 ° C./s
In addition, after cooling to T ₁ , rapid diffusion to below the M _S point suppresses the diffusion of C from the portion where the γ phase is generated, and hard martensite is generated to ensure high strength. Therefore, as a result of examining the temperature range and the cooling rate at which such controlled cooling should be performed, it was found that it is preferable to set the average cooling rate V ₃ in the temperature range from 700 ° C. to at least 400 ° C. to 10 ° C./s or more. did. However, when the cooling rate exceeds 80 ° C./s, the shape of the product deteriorates. For this reason, the average cooling rate V ₃ in the temperature range from 700 ° C. to at least 400 ° C. is preferably 10 to 80 ° C./s.

Here, the reason why the controlled cooling temperature range is limited to a temperature range from 700 ° C. to at least 400 ° C. is as follows.
That is, the temperature at which transformation from the γ phase to the α phase begins is around 700 ° C, and the temperature at which martensite begins to form is around 400 ° C, so cooling in the temperature range from 700 ° C to at least 400 ° C. This is because an appropriate amount of martensite is secured by appropriately controlling the speed.

  The temperature range from 400 ° C. to room temperature is not particularly limited, and it may be cooled at the same cooling rate or may be changed.

Furthermore, it is preferable to add a tempering treatment that is maintained at 200 to 400 ° C. for 30 to 2000 seconds after the rapid annealing of the continuous annealing. Thereby, the hard martensite phase is tempered, and the total elongation El and the hole expansion ratio λ are improved.
Moreover, after annealing, a chemical conversion process property improves by removing the surface oxide layer and a foreign material by pickling process. Moreover, yield point elongation can be suppressed by performing skin pass rolling with a rolling reduction of about 0.1 to 2%.

Steel with the composition shown in Table 1 is melted, heated to 1250 ° C, hot rolled to a 3.5 mm hot rolled steel sheet, pickled, and then cold rolled to a 1.4 mm cold rolled steel sheet. did. The cold-rolled steel sheet is heated and cooled under the conditions shown in Table 2-1 and Table 2-2. After reaching 250 ° C., it is subjected to continuous annealing that is held at 250 ° C. for 240 seconds, and then pickling and skin pass. Rolled into a product plate. In addition, Ac ₁ point and Ac ₃ point in Table 1 were calculated | required by following Formula, respectively.
Ac ₁ = 723-10.7 [% Mn] -16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr]
Ac ₃ = 910-203 [% C] ^0.5 +44.7 [% Si] -30 [% Mn] +700 [% P] +400 [% Al] +400 [% Ti]
+104 [% V] +32.5 [% Mo]
However, [% M] represents the content (mass%) of the M element.
From the product plate thus obtained, JIS No. 5 specimens were sampled in the direction perpendicular to the rolling direction, and the mechanical properties were investigated. The obtained results are shown in Table 2-1 and Table 2-2.

As is apparent from Tables 2-1 and 2-2, all of the product plates obtained according to the present invention not only satisfy the range where the tensile strength is 780 MPa or more and the yield ratio is 0.60 to 0.90. The elongation El is 22% or more for the tensile strength of 780 MPa class and 15% or more for the 980 MPa class, indicating that high strength, low yield ratio and high total elongation are achieved at the same time. Moreover, it has been confirmed that all of the obtained product plates are excellent in chemical conversion treatment and weldability.
Further, when V ₀ is controlled within the range of the above formula (2), V ₂ is controlled within the range of the above formula (3), and V ₃ is controlled within the range of 10 to 80 ° C./s, the tensile strength is 780 MPa class and 24 %, 980MPa class with 17% or more total elongation and yield ratio reduced to 0.60 ~ 0.85.
On the other hand, in any of the comparative examples, any of the tensile strength, the yield ratio, and the total elongation could not satisfy the desired characteristics.

Claims (4)

% By mass
C: 0.05 to 0.12%,
Si: 0.5% or less,
Mn: 2.0-4.0%
Ti: 0.005-0.06%,
Nb: 0.005-0.08% and
Al: A steel slab containing 0.1% or less, the balance being Fe and inevitable impurities, is hot-rolled, pickled, cold-rolled, and then subjected to temper rolling after continuous annealing. In the method of manufacturing a cold-rolled steel sheet comprising:
In the temperature increase process of the continuous annealing, the average temperature increase rate V ₁ in the temperature range from 700 ° C. to the steel plate temperature T ₁ (° C.) defined by the following formula (1) is 0.3 to 8 ° C./s,
A method for producing a high-strength cold-rolled steel sheet excellent in workability, characterized in that the maximum temperature T _M in the continuous annealing is in the range of Ac ₁ point or more and less than Ac ₃ point.
T ₁ = 0.98T _M (1)
However, the T _M is the maximum temperature of the steel sheet in the continuous annealing (° C.). 2. The processing according to claim 1, wherein in the temperature raising process of the continuous annealing, an average temperature rising rate V ₀ in a temperature range from 300 ° C. to at least 650 ° C. is set in a range defined by the following formula (2). A method for producing a high-strength cold-rolled steel sheet having excellent properties.
Record
2.0 ・ V ₁ ≦ V ₀ ≦ 20 ・ V ₁ (2) In the cooling process of the continuous annealing, the average cooling rate V ₂ in the temperature range from the maximum temperature T _M to the steel plate temperature T _{1 is set} to a range represented by the following formula (3),
Method for producing a high strength cold rolled steel sheet having excellent workability according to claim 1 or 2 Average cooling rate V _3, characterized in that a 10 to 80 ° C. / s in the temperature range up to at least 400 ° C. from 700 ° C. .
Record
{0.5 / (V ₁ +0.3)} + 0.3 ≦ V ₂ ≦ {3 / (V ₁ +1)} + 0.7 (3) The steel slab is further mass%,
B: 0.0005 to 0.0030%,
Mo: 0.05-2%
V: 0.05-0.5% and
Cr: 0.01-1%
The method for producing a high-strength cold-rolled steel sheet having excellent workability according to any one of claims 1 to 3, comprising one or more selected from among the above.

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