JP3450985B2 - High-strength cold-rolled steel sheet having good shape and excellent bendability and manufacturing method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength cold-rolled steel sheet having a good shape and excellent bendability, and a method for producing the same. The cold-rolled steel sheet according to the present invention is used for press forming in applications such as automobiles, household electric appliances, and construction, and is a cold-rolled steel sheet in a narrow sense that is not surface-treated, and for rust prevention, for example, It includes both a surface treatment such as Zn plating and alloyed Zn plating, and further a cold-rolled steel sheet having an organic film treatment thereon.

[0002]

2. Description of the Related Art With the increasing number of automobile accidents and social problems as motorization progresses, it becomes necessary to reduce the number of accidents and prevent passengers from being injured by accidents. Adopting reinforcement members such as door guard bars. Has advanced. In addition, it became necessary to strengthen the functions of the bumper beam, etc., but increasing the plate thickness of existing parts at the same time as the adoption of new parts will increase the weight of the automobile body, leading to an increase in fuel consumption and exhaust gas. In addition to the rise in oil prices and the crisis in the supply system, the danger of the destruction of the global environment has recently become a concern.

Under such circumstances, it has been considered that the strength of the steel sheet should be increased to reduce the thickness of the component, and the development of a high-strength steel sheet having workability capable of imparting a shape as an automobile component has been advanced. Among them, for example, ferritic / martensitic duplex stainless steels as described in Japanese Patent Publication No. 56-11741, so-called Dual-Phase steels, for example, Japanese Patent Publication No. 6-3.
Composite structure steels containing retained austenite as disclosed in Japanese Patent No. 5619 are prominent, but when any of the steels is made to have a tensile strength exceeding 980 MPa which is required as a reinforcing component, bendability deteriorates, Even if an attempt is made to perform spot welding in order to assemble a vehicle body, it is difficult to form a sound welded portion, resulting in lack of reliability.

On the other hand, for example, JP-A-62-1353
Japanese Patent Publication No. 3 proposes increasing the homogeneity of the structure in order to make the steel sheet having a tensile strength of 780 to 1470 MPa excellent in bendability, and bainite transformation is comparatively performed in order to be subjected to severe bending work. It is recommended to perform at low temperature, but at low temperature, bainite transformation does not complete in a short time, so unstable austenite with extremely poor stability to processing remains in the metal structure, and bending contrary to the intention. May deteriorate.

On the other hand, for example, in Japanese Examined Patent Publication No. 2-35013, steel having a relatively small amount of C, Si, and Mn added is water-quenched from the annealing temperature and then tempered at 300 ° C. or less to obtain a high strength of 780 MPa or more. Although a method of manufacturing a steel sheet is disclosed, in this case, it is difficult to uniformly cool the entire steel sheet, and there is a problem that the shape is likely to be defective.

[0006]

As described above, in a steel sheet having a metal structure mainly composed of bainite or tempered martensite and having a tensile strength of 780 to 1470 MPa, a steel sheet having a good shape and excellent bendability, It has been an issue to provide a manufacturing method thereof.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, in a steel sheet having a metal structure mainly composed of bainite, cooling was performed at a predetermined bainite transformation temperature. Even if some tempered martensite is mixed by reheating after supercooling without finishing or bainite having different hardness due to transformation at different temperature exists, Ms point is -196 ° C or more. If the volume ratio of retained austenite is 2% or less,
In a steel sheet having a tensile strength of 780 to 1470 MPa, bendability is not practically deteriorated as compared with the case where cooling is terminated at a predetermined bainite transformation temperature, and comparison is made with the case of cooling to room temperature and then reheating. Then, it was found that the shape can be remarkably good.

The present invention is an unprecedented new steel plate constructed on the basis of such an idea and new knowledge, and the gist thereof is as follows. (1) C: 0.13 to 0.20% by weight, Si:
0.6% or less, Mn: 1.8 to 2.8%, P: 0.02
% Or less, S: 0.015% or less, Al: 0.005-
0.1%, N: 0.0060% or less, balance Fe
And a high-strength cold-rolled steel sheet having a good shape and excellent bendability, characterized in that the metallographic structure of the steel consisting of unavoidable impurities contains residual austenite having an Ms point of -196 ° C or more in a volume ratio of 2% or less. . (2) Mo: 0.01 to 0.15% by weight, B:
A high-strength cold-rolled steel sheet containing at least one of 0.0005 to 0.0020% and having a good shape as described in (1) and excellent bendability.

(3) C: 0.13 to 0.20 by weight%
%, Si: 0.6% or less, Mn: 1.8 to 2.8%,
P: 0.02% or less, S: 0.015% or less, Al:
A slab containing 0.005 to 0.1% and N: 0.0060% or less and having a composition consisting of balance Fe and unavoidable impurities is finish-rolled at a temperature of Ar ₃ points or higher, and 500 ° C or higher and 750 ° C or lower. It is rolled up at the temperature of
~ 80% cold rolling, then 7 by continuous annealing equipment
500 for 30 seconds or more within a temperature range of 50 ° C to 900 ° C
After heating for less than or equal to seconds, the average cooling rate from 650 ° C to 350 ° C is set to 50 ° C / sec or more and 200 ° C / sec or less, and 200
To 300 ° C., and after maintaining the temperature range above the cooling end point temperature and 20 ° C. higher than the cooling end point temperature for 3 seconds or more and 60 seconds or more, 50 ° C. or more higher than the cooling end point temperature,
After reheating to a temperature of 400 ° C or less, 150 seconds or more and 90 seconds
Cooling to 200 ° C. or lower within 0 seconds, and containing a retained austenite having an Ms point of −196 ° C. or higher contained in the metallographic structure in an amount of 2% or less by volume ratio, a good shape and excellent bendability Of high strength cold rolled steel sheet. (4) Mo: 0.01 to 0.15% by weight, B:
The method for producing a high-strength cold-rolled steel sheet, which contains at least one of 0.0005 to 0.0020% and has a good shape and excellent bendability as described in (3) above.

The present invention will be described in detail below. First,
The reason for limiting the numerical values of C, Si, Mn, P, S, Al, N, Mo, and B will be described. C is an essential element when strengthening the steel sheet by strengthening the structure with bainite or tempered martensite. If C is less than 0.13%, continuous annealing is required to obtain the required tensile strength. It becomes necessary to cool the steel strip at a cooling rate that makes it difficult to secure the shape. On the other hand, if C exceeds 0.20%, it becomes difficult to form a sound welded portion by spot welding, and at the same time segregation of C becomes significant, resulting in deterioration of workability.

Si is known as an element that increases the strength of the steel sheet without significantly impairing the workability, especially elongation, and its addition is considered to be generally useful, but it has the effect of delaying the bainite transformation, and its addition amount is If it exceeds 0.6%, even if the cold-rolled steel strip is subjected to the heat treatment according to the present invention, its metal structure has a relatively high Ms point, and it is easily transformed into martensite even by a slight work and bendability. Since the volume ratio of retained austenite, which is a cause of damage to the steel, rapidly increases, the upper limit is set to 0.6%. Since Mn increases the hardenability of austenite together with C, 1.8
% Or more. However, if the added amount is too large, cracks are likely to occur in the slab and the spot weldability deteriorates, so the upper limit is 2.8%.

P is generally contained in steel as an unavoidable impurity, but if the amount exceeds 0.02%, in a high strength steel sheet having a tensile strength of more than 780 MPa as in the present invention, toughness and cold rolling are involved. Remarkably deteriorates the sex. S
Is generally contained in steel as an unavoidable impurity, but if the amount exceeds 0.015%, MnS elongated in the rolling direction is produced.
Is significantly present, which adversely affects the bendability of the steel sheet.

Al is used as a deoxidizing element of steel, and in order to improve the material quality by suppressing the grain refinement of the hot rolled material by AlN and the coarsening of crystal grains in a series of heat treatment steps.
It is necessary to add more than 05%, but if it exceeds 0.1%, not only the cost becomes high, but also the surface properties are deteriorated.
N is also generally contained in steel as an unavoidable impurity,
If the amount exceeds 0.060%, brittleness deteriorates along with elongation, so this is made the upper limit.

Mo and B are generally known as elements that increase hardenability, and may be added for the purpose of suppressing the formation of ferrite during cooling and preventing the decrease in tensile strength. If the addition amount of Mo is less than 0.01% or the addition amount of B is less than 0.0005%, the effect is not recognized. Mo exceeding 0.15% and 0.0020%
Addition of more than B causes ear cracking and makes hot rolling and cold rolling difficult, so it must be avoided. Nb, Ti, Cu, Sn, Zn, Zr, W, C in steels containing these as main components
Even if r and Ni are contained in a total amount of 1% or less, the effect of the present invention is not impaired, and depending on the amount, it may be preferable that the corrosion resistance is improved.

Next, the reasons for limiting the manufacturing conditions will be described. The slab used for hot rolling is not particularly limited. That is, it may be manufactured by a continuously cast slab or a thin slab caster. It is also suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting. The finishing temperature of hot rolling needs to be Ar ₃ or higher from the viewpoint of not deteriorating the shape of the steel strip. The coiling temperature after hot rolling is 550 ° C. or higher and 750 ° C. or lower. When the winding temperature exceeds 750 ° C., a band-shaped metal structure is formed and Mn is concentrated in the iron carbide,
After cold rolling, it is not easily decomposed even if heated to 750 ° C. or higher, and not only the bendability deteriorates after the series of heat treatments, but also the tensile strength decreases. On the other hand, if the winding temperature is less than 550 ° C, the cold workability deteriorates.

Cold rolling may be performed under ordinary conditions, and the rolling ratio is 50% or more for the purpose of refining the metal structure after completion of a series of heat treatments and obtaining excellent bendability. On the other hand, 80%
It is not realistic to perform cold rolling at a rolling ratio exceeding 0.1% because a large amount of cold rolling load is required. After cold rolling, when performing a series of heat treatment with continuous annealing equipment, first, 750 ° C or higher and 900
After heating for 30 seconds or more and 500 seconds or less to a temperature of ℃ or less, and after a series of subsequent heat treatments is completed, a metal structure mainly composed of bainite or tempered martensite is used, so that the volume ratio is 80%.
% Or more austenite is generated. Heating temperature is 75
When the temperature is lower than 0 ° C, not only austenite is insufficiently formed, but also recrystallization is insufficient, so that the steel sheet cannot be provided with the formability necessary for elongation and bendability. When the heating time is less than 30 seconds, a sufficient volume ratio of austenite is not generated when the heating temperature is low.

On the other hand, heating to a temperature exceeding 900 ° C. requires only excessive energy, and the effect commensurate with that is not seen, and heating for a time exceeding 500 seconds requires continuous annealing equipment. Either the line length must be too long or the threading speed must be significantly reduced, but the effect commensurate with it must be obtained. After heating, when cooling to a predetermined bainite transformation or lower, the cooling rate is 50 ° C./sec or more and 200 ° C. at a cooling rate from 650 ° C. to 350 ° C.
/ Sec or less. If this cooling rate is less than 50 ° C / sec,
Since the austenite generated during heating transforms into ferrite or pearlite, it is not possible to obtain a metallic structure mainly composed of bainite or tempered martensite after the subsequent series of heat treatments, resulting in a decrease in tensile strength or tensile strength. Even if the value does not decrease, the formability required for the steel sheet including elongation and bendability deteriorates. On the other hand, at a speed exceeding 200 ° C./sec, it is difficult to uniformly cool the entire steel sheet, and the shape tends to be defective.

In the present invention, the cooling is completed at 200 to 300 ° C., and the temperature is equal to or higher than the cooling end point temperature and is lower than the cooling end point temperature 20.
After keeping the temperature range higher than ℃ for 3 seconds or more and 60 seconds or less, and then reheating to a temperature that is 50 ℃ or more higher than the cooling end temperature and 400 ℃ or less, 200 ℃ within 150 seconds or more and 900 seconds.
It is cooled to the following, but in the metallographic structure of the steel of the present invention, the volume fraction of retained austenite having an Ms point of −196 ° C. or higher is 2% or less, and the tensile strength is 780 to 1470 MPa.
In the present invention, which is an object of the present invention to provide a steel sheet excellent in bendability and a method for producing the steel sheet, it is a very important heat treatment condition, and by cooling to a temperature lower than a predetermined bainite transformation temperature by 50 ° C. or more, austenite is changed. By accelerating the transformation to bainite or martensite, the amount of Si contained in the steel is set to 0.6% or less, and the bainite transformation is quickly completed at a predetermined transformation temperature in a short time. With the goal.

When the cooling end temperature is lower than 200 ° C., the volume ratio of austenite transformed into martensite rapidly increases, and the shape of the steel sheet becomes defective. On the other hand, if the cooling is completed at a temperature higher than 300 ° C., the line length of the continuous annealing equipment must be excessively long before the reheating, or must be maintained for a time such that the strip running speed must be remarkably reduced. However, the effects characteristic of the present invention do not appear. If the holding time after cooling is less than 3 seconds or if it is maintained at a temperature higher than 20 ° C. higher than the cooling end temperature, the time required for bainite transformation cannot be shortened, and the volume ratio exceeds 2% M
The retained austenite having a s point of −196 ° C. or higher is likely to be included in the metal structure, and the bendability of the steel sheet deteriorates. Further, in order to maintain the time for more than 60 seconds, it is uneconomical to make the line length of the continuous annealing equipment too long or to significantly reduce the strip running speed.

Unless the reheating temperature is higher than the cooling end temperature by 50 ° C. or more, the effects of shortening the bainite transformation time and reducing the volume ratio of retained austenite whose Ms point contained in the metal structure is −196 ° C. or more are not clear. Absent. On the other hand, 40
When reheated to a temperature exceeding 0 ° C., the tensile strength does not reach the object of the present invention, 780 to 1470 MPa.
Also, after reheating, the time to cool down to 200 ° C or less is 1
If it is less than 50 seconds, the bainite transformation will be completed even if it is cooled to a temperature lower than the predetermined bainite transformation temperature, which is a feature of the present invention, by 50 ° C. or more and the transformation from austenite to bainite or martensite is promoted. However, the metal structure is likely to contain retained austenite having an Ms point of more than 2% in volume ratio of −196 ° C. or higher.

On the other hand, if the cooling time to 200 ° C. or lower exceeds 900 seconds, the line length of the continuous annealing equipment must be too long or the strip running speed must be remarkably reduced, which is uneconomical. . After this, if shape correction is required, temper rolling with an elongation of 0.3 to 1.5% is performed, and for rust prevention, for example, surface treatment such as Zn plating or alloyed Zn plating, Even if it is subjected to an organic film treatment or the like, it does not affect the good shape and the excellent bendability which are features of the present invention.

[0022]

EXAMPLES Next, examples of the present invention will be described with reference to examples. A slab having the composition shown in Table 1 was heated to 1150 ° C. to form a hot-rolled steel strip having a finishing temperature of 920 ° C. and a thickness of 4.5 mm.
It was wound up at 0 ° C. After pickling, cold rolling with a reduction rate of 64% was performed to make a cold rolled steel strip with a thickness of 1.6 mm, and then heat treatment under the conditions shown in Table 2 was performed using continuous annealing equipment to obtain a shape. Temper rolling was performed when straightening was required. JIS No. 5 test pieces are cut out from the steel strip thus manufactured in a direction perpendicular to the rolling direction and subjected to a tensile test at room temperature to give a yield strength (YP) and a tensile strength (T
S) and elongation (El) were determined, and strip test pieces were cut out in a direction perpendicular to the rolling direction and subjected to a 90 ° bending test to determine the critical bending radius. Further, the volume ratio of retained austenite before and after immersion in liquid nitrogen at -196 ° C for 1 hour was measured by an X-ray diffraction method from the surface layer along the center of 1/4 of the plate thickness, and the Ms point remained at -196 ° C or higher. The volume ratio of austenite was calculated.

[0023]

[Table 1]

The above results are shown in Table 2. As is clear from this table, sample No. which is the sample of the present invention. 3, 8-1
0, 17, and 19 have a metal structure mainly composed of bainite and tempered martensite, and the volume fraction of retained austenite having an Ms point of −196 ° C. or higher contained in the metal structure is 2% or less, and Strength is 780-1470M
At Pa, the shape is good and the bendability is excellent. In contrast,
Sample No. In the case of the component steels of the present invention, such as 2, 4 to 7, 11 to 15, 18, and 20, even if there is a condition out of the scope of the claims of the present invention in a series of heat treatment conditions, , Sample no. In the steels other than the components of the present invention such as 1, 16, 21, and 22, the tensile strength is 780 to 14
It does not come out of the range of 70 MPa, does not have a metal structure mainly composed of bainite or tempered martensite, and the volume ratio of retained austenite whose Ms point contained in the metal structure is −196 ° C. or more exceeds 2%, Strength is 780
Even at 1470 MPa, problems such as poor shape and poor bendability occur.

[0025]

[Table 2]

[0026]

As described above in detail, the present invention has a metal structure mainly composed of bainite and tempered martensite, and a steel sheet having a tensile strength of 780 to 1470 MPa has a good shape and excellent bendability. The present invention provides a steel sheet and a manufacturing method thereof, which contributes to the preservation of the global environment through the improvement of collision safety of automobiles and the reduction of fuel consumption and exhaust gas by reducing the weight.

[Brief description of drawings]

FIG. 1 is a diagram showing a heat treatment cycle performed on a steel sheet after cold rolling.

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Claims (4)

(57) [Claims] 1. C: 0.13 to 0.20% by weight,
Si: 0.6% or less, Mn: 1.8 to 2.8%, P:
0.02% or less, S: 0.015% or less, Al: 0.0
05-0.1%, N: 0.0060% or less, and M in the metallic structure of the steel consisting of the balance Fe and unavoidable impurities.
A high-strength cold-rolled steel sheet having a good shape and excellent bendability, which contains 2% or less by volume of retained austenite having an s point of −196 ° C. or more. 2. Mo: 0.01-0.15 by weight.
%, B: 0.0005 to 0.0020%, a high-strength cold-rolled steel sheet having a good shape and excellent bendability according to claim 1. 3. C: 0.13 to 0.20% by weight,
Si: 0.6% or less, Mn: 1.8 to 2.8%, P:
0.02% or less, S: 0.015% or less, Al: 0.0
A slab having a composition of 0.05 to 0.1%, N: 0.0060% or less, and a balance of Fe and inevitable impurities is A
r Finish rolling at a temperature of ₃ points or more, 500 ° C or more 75
Rolled at a temperature of 0 ° C or less to form a hot rolled steel strip, 50 to 80
% Cold-rolled, then 750 ℃ with continuous annealing equipment
After heating in the temperature range of 900 ° C. or higher for 30 seconds or more and 500 seconds or less, the average cooling rate from 650 ° C. to 350 ° C. is set to 50 ° C./second or more and 200 ° C./second or less, and 200 to 30
After cooling to 0 ° C. and further maintaining the temperature range higher than the cooling end point temperature and 20 ° C. higher than the cooling end point temperature for 3 seconds to 60 seconds, the temperature is higher than the cooling end point temperature by 50 ° C. or more,
After reheating to a temperature of ℃ or less, cool to 200 ℃ or less within 150 seconds to 900 seconds, and contain 2% or less by volume of retained austenite having a Ms point of -196 ℃ or more contained in the metal structure. A method for producing a high-strength cold-rolled steel sheet having a good shape and excellent bendability, characterized by: 4. Mo: 0.01 to 0.15 by weight.
%, B: 0.0005 to 0.0020%, and a method for producing a high-strength cold-rolled steel sheet having a good shape and excellent bendability according to claim 3.