JP3371746B2 - High formability, high tensile strength cold rolled steel sheet for automobile body strength members and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high formability, high strength cold rolled steel sheet suitable for a strength member of an automobile body and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, as a high-strength steel sheet used for drawing, an IF steel obtained by adding Ti or the like to an ultra-low carbon steel as proposed in JP-A-7-216452 is used.
Those to which solid solution strengthening elements such as i, Mn and P are added have been put into practical use.

However, since this steel has a clean ferrite grain boundary and a large amount of solid solution strengthening type alloying elements such as Si and Mn are added, the formability is excellent, but the reliability as a strength member after forming is high. Lack.

For this reason, the solid solution strengthened IF steel high-strength cold-rolled steel sheet is usually used only for members which are required to have high formability but are not so stressed after forming. It is not used as a representative strength member for automobile bodies.

Japanese Patent Laid-Open No. 7-90482 proposes a steel sheet having excellent impact resistance as such a steel sheet for strength members of automobiles. However, 0.01% or more of C is added to this steel sheet. Therefore, it cannot be said that the formability is as excellent as that of IF steel.

[0006]

As described above, the steel based on IF steel containing a large amount of solid solution strengthening type alloying elements such as Si and Mn disclosed in JP-A-7-216452 is excellent. Although it has excellent moldability, it is not suitable for use as a strength member for automobile bodies. Furthermore, this steel is a solid solution strengthened high-strength cold-rolled steel sheet based on IF steel in which Ti and Nb are added as carbonitride-forming elements in a complex manner. The temperature range is narrowed, and the moldability is reduced and the slab cost is increased.

Therefore, a high workability, high tensile cold rolled steel sheet having strength as a steel sheet for automobile body strength members while reducing solid solution strengthening alloy elements such as Si and Mn to maintain excellent formability. However, such a steel plate has not been proposed yet.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a high-strength cold-rolled steel sheet having excellent formability and mechanical properties for a strength member of an automobile body and a method for producing the same. To aim.

[0009]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to solve the above-mentioned problems. As a result, Si,
In order to reduce the addition amount of alloying elements such as Mn and obtain a high-strength cold-rolled steel sheet having excellent formability and mechanical properties for strength members of automobile bodies, (1) optimize C and Ti contents. (2) As a method of precipitation strengthening, precipitation strengthening by NbC is not preferable, and therefore, a Ti addition system is preferable as the steel type (3) Si, Mn, and P Regarding the solid solution strengthening method, since the steel sheet targeted by the present invention is particularly used for automobile body strength members, it has been found that it is necessary to minimize the strengthening by P and mainly the strengthening by Si and Mn. It was

As a manufacturing method for obtaining the high-strength cold-rolled steel sheet suitable for the above-mentioned strength member for automobile body, the steel sheet is manufactured by a process including at least a hot rolling step, a cold rolling step and an annealing step. At this time, (1) the precipitation size of TiC is controlled by setting the coiling temperature during hot rolling within a specific range, and the effect of precipitation strengthening is utilized to the maximum. (2) Furthermore, the annealing temperature is within a specific range. By
It has been found that this can be achieved by making the tensile strength and yield ratio obtained after annealing suitable for automobile body strength members.

The present invention has been made on the basis of such findings, and provides the following (1) to (4). (1) C: 0.005-0.02%, S in weight%
i: 0.5% or less, Mn: 0.5 to 2.4%, P: 0.
01-0.05%, S: 0.003-0.012%, so
l.Al: 0.07% or less, N: 0.005% or less, T
i: 0.04 to 0.12%, the balance being substantially Fe
Consists of, and meet the 0.9 ≦ ^Ti * /C≦1.3,
Tensile strength is 400-500MPa and yield ratio is 65%
A high formability, high tensile cold rolled steel sheet for a strength member of an automobile body, characterized by the above.

(2) C: 0.005 to 0.
02%, Si: 0.5% or less, Mn: 0.5 to 2.4
%, P: 0.01 to 0.05%, S: 0.003 to 0.
012%, sol.Al: 0.07% or less, N: 0.005
% Or less, Ti: 0.04 to 0.12%, B: 0.000
3 to 0.002% , the balance consisting essentially of Fe
And satisfying 0.9 ≦ Ti ^{* /} C ≦ 1.3, tensile strength
Degree of 400 ~ 500MPa and yield ratio of 65% or more
A high-formability, high-strength cold-rolled steel sheet for strength members of automobile bodies, characterized by being present.

(3) C: 0.005 to 0.
02%, Si: 0.5% or less, Mn: 0.5 to 2.4
%, P: 0.01 to 0.05%, S: 0.003 to 0.
012%, sol.Al: 0.07% or less, N: 0.005
% Or less, Ti: 0.04 to 0.12% is contained, and the balance is actual.
Fe qualitatively, and 0.9 ≦ Ti ^{* /} C ≦ 1.3
When manufacturing a steel sheet satisfying the requirements by a process including at least a hot rolling process, a cold rolling process, and an annealing process, the coiling temperature during the hot rolling is 580 to 720 ° C., and the annealing temperature during the annealing is 740 to 740. 830 degreeC, The manufacturing method of the high formability high tension cold-rolled steel plate for the strength member of a motor vehicle body characterized by the above-mentioned.

(4) C: 0.005 to 0.
02%, Si: 0.5% or less, Mn: 0.5 to 2.4
%, P: 0.01 to 0.05%, S: 0.003 to 0.
012%, sol.Al: 0.07% or less, N: 0.005
% Or less, Ti: 0.04 to 0.12%, B: 0.000
3 to 0.002% , the balance consisting essentially of Fe
And heat the steel sheet satisfying 0.9 ≦ Ti ^{* /} C ≦ 1.3
At least includes hot rolling process, cold rolling process, and annealing process
Winding temperature during hot rolling when manufacturing in process
To 580 to 720 ° C., and the annealing temperature during annealing is 7
40-830 degreeC , The manufacturing method of the high formability high tension cold-rolled steel plate for the strength member of automobile bodies characterized by the above-mentioned. However,
The above Ti ^* satisfies the following formula. Ti ^* = (12/48) * {Ti- (48/14) * N- (48/32) * S}

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The high-strength cold-rolled steel sheet of the present invention has a tensile strength level of TS: 400 to 500 MPa.
In order to obtain a steel plate having excellent car body strength member suitability and required mechanical properties as a high-tensile cold-rolled steel plate,
As described above, by optimizing the amounts of C and Ti to maximize the use of precipitation strengthening by TiC, it is preferable to use Ti alone as a steel type, and further, strengthening by P should be minimized and by using Si and Mn. It is necessary to mainly strengthen, but in addition, when excellent impact absorption capacity is required for automotive materials, increase the yield strength and secure a yield ratio of 65% or more in order to increase the impact absorption energy. It is important to.

For this reason, in the present invention, in% by weight, C:
0.005-0.02%, Si: 0.5% or less, Mn:
0.5-2.4%, P: 0.01-0.05%, S:
0.003 to 0.012%, Sol. Al: 0.07%
Hereinafter, N: 0.005% or less, Ti: 0.04 to 0.1
2%, 0.9 ≦ Ti ^{* /} C ≦ 1.3, tensile strength of 400 to 500 MPa and yield ratio of 6
5% or more.

The reasons for limiting these chemical compositions will be described below. C: 0.005-0.02% C is an element to be reduced from the viewpoint of workability,
In the present invention, since strengthening by precipitation of TiC is essential, the range is 0.005-0.02%. 0.00
The range of 8 to 0.015% is more preferable.

Ti: 0.04 to 0.12% Ti is an essential additional element from the viewpoint of utilizing precipitation strengthening by TiC in the present invention. However, when the Ti content is 0.
If it is less than 04%, the effect intended by the present invention cannot be obtained, while if it exceeds 0.12%, the workability deteriorates. Therefore, the Ti amount is set to 0.04 to 0.12%.

Ti ^* / C ^: 0.9 to 1.3 Ti ^* / C is an important parameter for sufficiently exerting the effect of precipitation strengthening by TiC intended in the present invention and for ensuring excellent workability. Is. Here, Ti ^* is represented by the following formula. Ti ^* = (12/48) × {Ti− (48/14) × N
When-(48/32) * S} Ti ^* / C is less than 0.9, not only the effect of precipitation strengthening by TiC cannot be sufficiently obtained, but also the aging resistance deteriorates due to the residual solid solution C. On the other hand, if it exceeds 1.3, although the aging resistance is good, TiC precipitated in the steel is coarse and sparsely dispersed, so that the yield strength is reduced and as a result the yield ratio is reduced to less than 0.65. . Therefore, Ti ^* / C is 0.9-
Set to 1.3.

FIG. 1 shows the relationship between Ti ^* / C and the aging index and yield ratio. The aging index is JIS5 from the annealed sheet.
No. 4 specimens were cut and prestrained to 8%, then 100
It was quantified by the rising stress when heat treatment was carried out at 1 ° C. for 1 hour.
On the other hand, the yield ratio was calculated as (yield strength) / (tensile strength) when a JIS No. 5 test piece was cut out from the annealed plate and a tensile test based on JIS Z2241 was performed. The TiC size contributing to precipitation strengthening in the steel of the present invention is 100 nm or less.

Si: 0.5% or less Si has the effect of strengthening the steel sheet by solid solution, but it is preferable that Si is small because it lowers the workability. If the amount of Si exceeds 0.5%, the workability deteriorates significantly, so the content is made 0.5% or less. The surface treatability is
Has a strong effect, so that the amount of Si is preferably 0.2% or less particularly for the purpose of hot dip galvanizing.

Mn: 0.5-2.4% Mn has the function of strengthening the steel sheet by solid solution, and the unit wt%
Since the amount of deterioration of ductility per hit is smaller than that of Si and P,
It is preferable that the solid solution strengthening is mainly made of Mn for use as a strength member for automobile bodies. In the present invention steel, precipitation strengthening by TiC is utilized, but solid solution strengthening by Mn is also important, so M
n must be at least 0.5%. On the other hand, if it exceeds 2.4%, the surface quality is deteriorated more than the strength secured by Mn, and the Ar ₃ point is lowered to narrow the annealing temperature range. Therefore, the Mn content is set to 0.5 to 2.4%.

P: 0.01-0.05% P has a function of solid-solution strengthening with a small amount, but it is preferable that the amount is small because grain boundary embrittlement easily occurs due to grain boundary segregation. However, it is not preferable to set the amount to less than 0.01% from the viewpoint of economical efficiency during melting. On the other hand, 0.05
If it exceeds 0.1%, grain boundary embrittlement due to grain boundary segregation becomes significant. Therefore, the content of P is set to 0.01 to 0.05%.
In the case of the present invention, since a steel plate for a strength member of an automobile body is particularly intended, it is required that the steel plate characteristics in the application, that is, no crack when an impact stress is applied, no press crack due to P segregation, etc. Considering this, the upper limit is particularly strict.

S: 0.003 to 0.012% S is preferably reduced, but if the amount is less than 0.003%, the peelability of the hot ductile scale decreases. On the other hand, 0.
If it exceeds 012%, the ductility that the high-strength steel sheet should have is reduced. Therefore, the S content is 0.003 to 0.
012%.

Sol.Al: 0.07% sol.Al is used as a deoxidizing agent for steel, but if it is added in a large amount, the cost increases, so the content is made 0.07% or less.

N: 0.005% or less N is economical because the smaller the content of N, the smaller the amount of the carbonitride forming element described later. N amount is 0.005
If the content exceeds%, even if a carbonitride forming element is added and N is fixed, deterioration of the workability of the steel sheet cannot be avoided. Therefore, N
The content is 0.005% or less. A more preferable range for improving workability is 0.003% or less.

B: 0.0003 to 0.002% In the present invention, B may be added in the range of 0.0003 to 0.002% for the purpose of improving the secondary work embrittlement resistance. However, if the B content is less than 0.0003%, the effect of improving the secondary work embrittlement resistance cannot be obtained, while if it exceeds 0.002%, the effect is saturated. Therefore, when B is added, its content should be 0.0003 to 0.002.
%.

Next, a manufacturing method for obtaining the steel sheet of the present invention will be described. In the present invention, when a steel sheet having the above composition is manufactured by a process including at least a hot rolling step, a cold rolling step, and an annealing step, the coiling temperature during hot rolling is set to 580 to 720 ° C., and further annealing is performed. The annealing temperature at that time is 740 to 830 ° C. The reasons for limiting these manufacturing conditions will be described below.

Winding temperature during hot rolling: 580 to 720 ° C. If the winding temperature during hot rolling changes, the size of the carbides changes, which affects the workability after cold rolling annealing. Control of temperature is important. Winding temperature is 580 ℃
If it is less than r, fine carbides are re-dissolved during annealing, the amount of dissolved C increases and the r value becomes low. On the other hand, if it exceeds 720 ° C., the crystal grain size after hot rolling is coarse and r The value becomes low.
Thus, since the workability (r value) after cold rolling annealing is at a favorable level in the winding temperature range of 580 to 720 ° C.,
The range of the winding temperature is 580 to 720 ° C. Considering the surface texture, it is preferable to set the coiling temperature to 680 ° C. or lower from the viewpoint of descaling the hot rolled sheet.

Annealing temperature: 740 to 830 ° C. The annealing temperature in recrystallization annealing performed after cold rolling is
It is important for obtaining the required mechanical properties intended in the present invention. If this temperature is lower than 740 ° C., recrystallization is not completed and required mechanical properties cannot be obtained. On the other hand, when the annealing temperature exceeds 830 ° C., the carbides in the steel are re-dissolved at the annealing temperature, the amount of dissolved C increases, the r value decreases, and the yield strength decreases, resulting in The yield ratio becomes smaller.
Therefore, the range of the annealing temperature is set to 740 to 830 ° C.

The recrystallization annealing may be carried out by any of continuous annealing, box annealing, and continuous heat treatment prior to hot dip galvanizing.

The cold rolling performed prior to the recrystallization annealing makes the steel sheet have a predetermined plate thickness and develops a rolling texture to obtain a preferable texture for improving the workability in the subsequent recrystallization annealing step. It is given to develop.
The conditions of cold rolling are not particularly limited, but for the above-mentioned purpose, it is preferable to work to a final plate thickness with a reduction rate of 50% or more.

The material steel having the above composition is melted by, for example, a converter or an electric furnace. The billet may be manufactured by any of the ingot-segmentation rolling method, the continuous casting method, the thin slab casting method, and the strip casting method. In the present invention, in the manufacturing method of heating a slab obtained by continuous casting, ingot casting, or slab rolling, the slab is cooled to a temperature of room temperature or higher and then charged into a hot rolling furnace. In that case, it is preferable for controlling the structure that the charging temperature to the hot rolling furnace is not higher than Ar ₃ point.

Although the hot rolling heating temperature is not particularly specified, the hot rolling finishing temperature is set to Ar ₃ from the viewpoint of the uniformity of the structure.
The heating temperature is preferably 1050 ° C. or higher, and the heating temperature required therefor is 1050 ° C. Further, the steel according to the present invention can be obtained by a method of directly hot rolling a continuously cast slab other than the above-mentioned method, thereby producing the slab at a lower cost than the production method of performing slab heating. be able to. Even in this case, it is preferable to set the hot rolling finishing temperature to Ar ₃ point or higher from the viewpoint of the uniformity of the structure.

The cold-rolled steel sheet according to the present invention may be appropriately subjected to surface treatment (chemical conversion treatment, hot-dip galvanizing, hot-dip galvanizing, electroplating, organic coating, etc.) for use.

[0036]

EXAMPLES Example 1 Next, specific examples according to the present invention will be described below in comparison with comparative examples. Steels (material Nos. 1 to 12) having the chemical composition shown in Table 1 are heated at 1200 ° C., finished at 900 ° C., wound at 620 ° C., descaled, and then subjected to 75% cold rolling. Implemented and continued 8
Annealing was performed at 10 ° C. in a thermal cycle simulating continuous annealing to obtain a thin plate sample having a plate thickness of 0.8 mm. The sheet sample subjected to 1.0% temper rolling to, YP is a typical indication of tensile properties, TS, yield ratio were evaluated ElAI and brittle transition temperature. The mechanical properties were determined by a JIS No. 5 tensile test piece taken at a right angle from the rolling direction. The brittleness transition temperature is set so that the cup sample prepared with a deep drawing condition of a drawing ratio of 2.1 (initial blank diameter: 105 mmφ) and a forming height of 35 mm is immersed in a refrigerant containing alcohol as a main component so that the cup end portion spreads. It was determined as the lowest temperature at which ductile change occurs when stress is applied to. The results are shown in Table 2.

As is clear from the results shown in Table 2, the required tensile strength (TS ≧ 400 MPa) and high formability (EI ≧ 30) of the materials according to the present invention (Steel Nos. 1 to 7).
%, R value ≧ 1.40) and the yield ratio, which is the most important index in the present invention, was confirmed to satisfy 0.65 or more. Material No. Nos. 1, 4, 5, and 7 are those in which B is added within the range of the present invention, but the brittle transition temperatures of these materials are those of material No. B to which B is not added. It was lower than those of Nos. 2, 3, and 6, and the effect of improving the secondary workability by adding B was confirmed.

On the other hand, in the case where the C content exceeds the upper limit of the range of the present invention, No. In No. 8 steel, El is less than 30% and r value is 1.
In addition to being as low as 26, the occurrence of solid solution C is remarkable, so that a high AI is exhibited. The No. of Ti ^* / C value is greater than the upper limit No. 9 steel having a yield ratio of less than 0.65 and less than the lower limit. The yield ratio was less than 0.65, and the r value was particularly low and the AI value was close to 40 MPa even with 10 steel.

On the other hand, although the range of the components specified in the present invention was satisfied, No. with Nb was added. In No. 11 steel, due to the addition of Nb, a low ductility of less than 30% and an r value of less than 1.3 were obtained. In addition, the Ti amount is less than the lower limit. In No. 12 steel, the yield ratio is less than 0.65 because the effect of precipitation strengthening by TiC is insufficient. From the above, it was confirmed that it is necessary to satisfy the composition of the present invention in order to obtain the required mechanical properties and the high-strength cold-rolled steel sheet suitable for the strength member of the automobile body.

(Example 2) Steel No. 1 shown in Table 1 3, 5,
After changing the winding temperature variously by using No. 7, cold rolling of 80% was performed to obtain a cold-rolled steel sheet of 0.75 mm.
Continuous annealing was performed at various annealing temperatures shown in. Using this annealed plate, the mechanical properties and brittle transition temperature were investigated in the same manner as in Example 1.

The results are shown in Table 3. From this table, it is clear that the characteristics of each steel deteriorate when the coiling temperature and the annealing temperature deviate from the ranges of the present invention.

Specifically, when the coiling temperature is lower than the lower limit, the TiC precipitated in the coiling step becomes fine, so that it is re-dissolved during annealing and the r value deteriorates (cases B and H). On the other hand, when the coiling temperature exceeds the upper limit, the precipitates become too coarse, so that the action of precipitation strengthening is insufficient and the yield ratio is 0.
It decreases to 60 (case F).

On the other hand, when the annealing temperature is below the lower limit, almost no grain growth occurs, so that the yield ratio can be secured, but the El and r values, which are indicators of ductility, are considerably low (case C). Further, when the annealing temperature exceeds the upper limit, the precipitated TiC re-dissolves by high temperature annealing,
Although the El and r values are good, the yield ratio is less than 0.65 (cases E and I).

As is clear from the above results, the mechanical properties required (TS ≧ 40) depending on the manufacturing conditions according to the present invention.
It was confirmed that 0 MPa, El ≧ 30%, r value ≧ 1.40 and yield ratio of 0.65 or more) were obtained.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

As described above, according to the present invention,
It is possible to provide a high-strength cold-rolled steel sheet suitable for an automobile body strength member and a method for producing the same, which brings industrially useful effects. In particular, the yield ratio of the material obtained by the present invention is 65% or more, which is higher than the yield ratio of the steel according to the prior art, and when excellent shock absorbing capacity is required for automobile materials, increase the shock absorbing energy. It also has the excellent effect that

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the Ti ^* / C value yield ratio and the aging index of the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-177281 (JP, A) JP-A-64-62440 (JP, A) JP-A-5-279748 (JP, A) JP-A-10- 265898 (JP, A) JP 10-265894 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. C: 0.005-0.02 in% by weight
%, Si: 0.5% or less, Mn: 0.5 to 2.4%,
P: 0.01 to 0.05%, S: 0.003 to 0.01
2%, sol.Al: 0.07% or less, N: 0.005% or less, Ti: 0.04 to 0.12%, and the balance substantially
Which is made of Fe , satisfies 0.9 ≦ Ti ^{* /} C ≦ 1.3, has a tensile strength of 400 to 500 MPa and a yield ratio of 65% or more, and is highly molded for strength members of automobile bodies. High strength cold rolled steel sheet. However, Ti ^* = (12/48) * {Ti- (48/14) * N- (48/32) * S} 2. C: 0.005-0.02 in% by weight
%, Si: 0.5% or less, Mn: 0.5 to 2.4%,
P: 0.01 to 0.05%, S: 0.003 to 0.01
2%, sol.Al: 0.07% or less, N: 0.005% or less
Below, Ti: 0.04 to 0.12%, B: 0.0003 to
Contains 0.002% and the balance consists essentially of Fe,
0.9 ≦ Ti ^{* /} C ≦ 1.3 and tensile strength is 4
A high formability, high tensile cold rolled steel sheet for a strength member of an automobile body , which has a yield ratio of 65% or more and is 00 to 500 MPa . However, Ti ^* = (12/48) * {Ti- (48/14) * N- (48/32 ) * S} 3. C: 0.005-0.02 in% by weight
%, Si: 0.5% or less, Mn: 0.5 to 2.4%,
P: 0.01 to 0.05%, S: 0.003 to 0.01
2%, sol.Al: 0.07% or less, N: 0.005% or less, Ti: 0.04 to 0.12%, and the balance substantially
When a steel sheet made of Fe and satisfying 0.9 ≦ Ti ^{* /} C ≦ 1.3 is manufactured by a process including at least a hot rolling step, a cold rolling step, and an annealing step, winding during hot rolling is performed. The taking temperature is 580 to 720 ° C., and the annealing temperature during annealing is 740 to 830 ° C.,
A method for manufacturing a high-formability, high-tensile cold-rolled steel sheet for strength members of automobile bodies . However, Ti ^* = (12/48) * {Ti- (48/14) * N- (48/32) * S} 4. In weight%, C: 0.005-0.02
%, Si: 0.5% or less, Mn : 0.5 to 2.4%,
P: 0.01 to 0.05%, S: 0.003 to 0.01
2%, sol.Al: 0.07% or less, N: 0.005% or less
Below, Ti: 0.04 to 0.12%, B: 0.0003 to
Contains 0.002% and the balance consists essentially of Fe,
Steel plate satisfying 0.9 ≦ Ti ^{* /} C ≦ 1.3
Process, cold rolling process, annealing process at least
The coiling temperature during hot rolling is 58
0-720 ℃, further annealing temperature during annealing 740-
830 degreeC, The manufacturing method of the high formability high tension cold-rolled steel plate for the strength member of a motor vehicle body characterized by the above-mentioned. However, Ti ^* = (12/48) * {Ti- (48/14) * N- (48/32 ) * S}