JPH07188856A - Cold rolled steel sheet excellent in delayed aging characteristic at ordinary temperature and baking hardenability - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet excellent in delayed aging characteristic at ordinary temp. and baking hardenability without causing age deterioration by adding small amounts of Mo to an ultralow carbon steel sheet in which solid-solution carbon is controlled to a specific small quantity. CONSTITUTION:A steel, having a composition which contains, by weight, <0.008% C, <0.8% Si, <1.5% Mn, <0.15% P, <0.020% S, 0.01-0.1% Al, <0.01% N, and 0.012-0.3% Mo and in which either or both of <0.13% Ti and <0.05% Nb are added, independently or in combination, according to inequality I and equations II and III (where Ti**=0 is satisfied when Ti** in the equations is <0), is used. This steel is hot-rolled and the resulting hot rolled plate is coiled, cold-rolled and drawn at >=75% draft, and annealed,by which the cold rolled steel sheet is formed. By this method, the cold rolled steel sheet, having 15-50ppm solid-solution carbon content and excellent in workability and baking hardenability and also delayed in age deterioration in spite of high baking hardenability, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet which is excellent in room temperature slow aging and bake hardenability.

[0002]

2. Description of the Related Art Conventionally, there have been attempts to improve bake hardenability by adding carbide forming elements such as Mo, W and Cr to ultra-low carbon steel. See, for example, Japanese Patent Application No. 4-9784. To be However, if the addition amounts of Mo, W, and Cr are reduced in order to obtain extremely high workability, it becomes difficult to secure a bake hardening amount of 50 N / mm ² or more. Thus, compatibility between workability and bake hardenability was not always sufficient.

[0003]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems and is excellent in workability and bake hardenability, and exhibits a slow aging deterioration despite a high bake hardenability. It is an object of the present invention to provide a cold-rolled steel sheet that is excellent in normal temperature aging and bake hardenability.

[0004]

The gist of the present invention is that, in% by weight, C: 0.008% or less, Si:
0.8% or less, Mn: 1.5% or less, P: 0.15%
Hereinafter, S: 0.020% or less, Al: 0.01-0.
1%, N: 0.01% or less, Mo: 0.012 to 0.
3%, with the balance Fe and unavoidable impurities,
One or two kinds of Ti and Nb are added in the range of Ti: 0.13% or less, Nb: 0.05% or less and satisfying the following formula, and 0.0050% ≧ C% − (12 / 48) × Ti ^* % ≧
0.0015% Ti ^* % = Ti ^** + (48/93) × Nb% Ti ^** % = Ti% − (48/14) × N% − (48/1
6) × S% However, when Ti ^** <0, it is characterized in that the amount of Ti ^** = O 2 solid solution C remains in the steel in the range of 15 to 50 ppm. Excellent cold rolled steel sheet.

The present invention will be described in detail below. The cold-rolled steel sheet targeted by the present invention is a so-called cold-rolled steel sheet not subjected to plating or the like, a plated steel sheet plated with zinc plating or the like. The manufacturing method thereof is not limited, such as a slab that is lumped after casting with a mold, or a slab that is cast by continuous casting.

The present inventors have conducted various studies in order to improve the relationship between the workability and the bake hardenability of the cold rolled steel sheet, and as a result, the amount of solid solution C in the steel has remained from 15 ppm to 50 ppm. 50N / m by adding a small amount of Mo to
It has been found that high workability can be obtained while ensuring bake hardenability of m ² or more. Further, it has been found that even if a large amount of solid solution C is left due to the action of Mo, the aging deterioration is less likely to occur.

The reasons for limiting the components of the present invention are as follows. C: The content of C is set to 0.008% or less. When it exceeds 0.008%, C is a strengthening element of steel, and the strength becomes high.
This is because the workability is impaired, and the amount of one or two elements of Ti and Nb added is increased, the strength increase due to precipitates is unavoidable, the workability is deteriorated, and it is economically disadvantageous. is there.

Si: Si is set to 0.8% or less because
This is because if it exceeds 0.8%, Si is a strengthening element of steel, the strength becomes high, and the workability is impaired. Mn: Mn is set to 1.5% or less because when it exceeds 1.5%, Mn is a strengthening element of steel, the strength becomes high and the workability is impaired. P: P is set to 0.15% or less because P is a reinforcing element of steel, and even if added in a small amount, the strength becomes high and the workability is impaired. Moreover, since P is an element that is likely to be concentrated in the crystal grain boundaries and cause grain boundary embrittlement, if added in excess of 0.15%, the workability is impaired.

S: S is set to 0.020% or less because S is
Is a meaningless element originally present in steel, and
If added in excess of 020%, if the amount of sulfide elements such as Mn is small, red hot embrittlement occurs during hot rolling and cracking occurs at the surface.
This is because so-called hot brittleness occurs. Al: 0.01 to 0.1% is 0.01
If it is less than 0.1%, deoxidation becomes insufficient and blowholes occur in the steel sheet, so that the quality as a steel sheet cannot be ensured. If it is added in excess of 0.1%, the strength becomes high and the workability is impaired. Is.

N: N is set to 0.01% or less because 0.
If it is added in excess of 01%, it becomes difficult to secure the aging property, and it becomes necessary to add a large amount of nitride forming elements such as Ti and Nb for securing the aging property, which increases the strength of the steel sheet and improves the workability. Is damaged. Mo: 0.01-0.3% Mo is 0.0
If it is less than 12%, when the amount of dissolved C in the steel is 15 ppm, the effect of exhibiting the room temperature delayed aging becomes small, and the room temperature delayed aging cannot be exhibited.
This is because if it is added in an amount exceeding 3%, the strength of the steel sheet increases and the workability is impaired.

Ti and Nb: Ti and Nb are T
i: 0.13% or less, Nb: 0.05% or less. This is because when the content of C is 0.0080%, which is the upper limit, the amount of solid solution C is 15 pp when added individually.
It is an upper limit value for ensuring m or more. Also, Ti and N
Although b can be added alone or in combination, it is added according to the following formula.

0.0050% ≧ C% − (12/48) ×
Ti^*% ≧ 0.0015% Ti^*% = Ti^**+ (48/93) x Nb% Ti^**% = Ti% − (48/14) × N% − (48/1
6) × S% where Ti^**When <0, Ti^**= O This is the amount of solid solution C in the steel, from 15ppm to 50ppm
It is for limiting the range. Solid solution C amount is 15p
If it is less than pm, the bake hardening amount is 50 N / mm ²The above
This is because it cannot be secured and high bake hardenability cannot be obtained,
Moreover, when it exceeds 50 ppm, when Mo is added
However, delayed aging at room temperature cannot be sufficiently secured, causing age hardening.
This is because. Therefore, the amount of solid solution C is limited to the above range.
However, as this method, Ti and N which are carbide forming elements are used.
Limit the amount of b.

It is considered that the reason why the delayed aging at room temperature is exhibited by the addition of Mo is that Mo suppresses the diffusion of C at around room temperature and delays the time until C is fixed to the mobile dislocation. Therefore, in the past, due to the problem of aging, high solid solution C
If the amount is set, stretcher strain etc. during pressing will occur and the quality will be impaired, the value as a product will be lost and it will not hold as a product, but by adding Mo, this can be avoided Becomes

In addition, although B is not limited to be added as an element to be contained, it may be added in an amount of about 0.0010% in order to improve secondary workability. The steel whose composition has been adjusted as described above is used as a cold-rolled steel sheet according to a conventional method. The finishing temperature of the hot rolling is preferably 900 ° C. or higher, but if it is 700 ° C. or higher, the rolling structure does not remain and the texture does not hinder the workability. .

Although hot rolling may be used as it is for applications requiring a thick plate, cold rolling is generally performed.
The cold rolling rate is preferably 50 to 90%. If it deviates from this range, the deep drawability deteriorates. After cold rolling, annealing is performed. The annealing temperature is preferably not lower than the recrystallization temperature and not higher than 950 ° C. If it is lower than the recrystallization temperature, the workability does not recover and the workability is poor, and if it exceeds 950 ° C., the crystal grains become coarse and the workability deteriorates rather in this case, so the above range is preferable.

After annealing, temper rolling is generally performed. If the shape is sufficient, temper rolling may not be performed. Although the above is the manufacturing process of a general cold-rolled steel sheet, the performance is the same even if it is manufactured in a hot dip galvanizing line that performs galvanization simultaneously with annealing. Further, the steel plate manufactured by the above cold-rolled steel plate manufacturing method can be electroplated to form a rust-proof steel plate.

[0017]

EXAMPLES Next, examples of the present invention will be shown together with comparative examples. Steel having the components shown in Table 1 was melted in a converter and continuously cast into a slab. This slab was manufactured under the conditions shown in Table 2 to obtain a cold rolled steel plate. Table 2 also shows the mechanical properties of these cold-rolled steel sheets.

The yield elongation at room temperature aging shown in Table 2 is the elongation at yield point measured by holding a tensile test in a constant temperature room at 30 ° C. for 6 months. In the present invention example, 50 N / mm
It can be seen that, despite exhibiting a high bake hardenability of ² or more, yield elongation does not occur at room temperature aging, indicating that it has excellent room temperature delayed aging. On the other hand, in Comparative Examples, when the bake hardening amount is high, the aging property is poor, and when the bake hardening amount is excellent, the bake hardening amount is low. In addition, the steel sheet of the example of the present invention has higher elongation than strength and has excellent workability.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

According to the present invention, it is possible to provide a cold-rolled steel sheet which is excellent in bake hardenability, room temperature delayed aging and workability, and contributes to weight reduction and quality maintenance of pressed parts such as automobile outer panels.

Claims (1)

[Claims] 1. In% by weight, C: 0.008% or less, Si: 0.8% or less, Mn: 1.5% or less, P: 0.15% or less, S: 0.020% or less, Al: 0.01 to 0.1%, N: 0.01% or less, Mo: 0.012 to 0.3%, balance Fe and inevitable impurities, Ti
And Nb are added in the range of Ti: 0.13% or less, Nb: 0.05% or less, and a range satisfying the following formula: 0.0050% ≧ C% − (12/48 ) × Ti ^* % ≧
0.0015% Ti ^* % = Ti ^** + (48/93) × Nb% Ti ^** % = Ti% − (48/14) × N% − (48/1
6) × S% However, when Ti ^** <0, it is characterized in that the amount of Ti ^** = O 2 solid solution C remains in the steel in the range of 15 to 50 ppm. Excellent cold rolled steel sheet.