JPH0649590A - Ultralow carbon steel free from deformation with the lapse of time at ordinary temperature and its production - Google Patents

Abstract

PURPOSE:To produce an ultralow carbon steel excellent in cold formability and free from deformation with the lapse of time at ordinary temp. by subjecting a steel having a specific composition consisting of C, Mn, S, Al, N, Ti, Nb, and Fe to hot rolling, to cold rolling, and then to continuous annealing under specific conditions. CONSTITUTION:A steel which has a composition consisting of, by mass, <=0.02% C, <=0.3% Mn, <=0.010% S, 0.01-0.1% Al, <=0.0030% N, further either or both of 0.01-0.08% Ti and 0.005-0.08% Nb, and the balance Fe with inevitable impurity elements and further containing, if necessary, 0.0001-0.0015% B is hot- rolled and cold-rolled by ordinary methods. Then the resulting steel stock is annealed so that annealing temp. ST in continuous annealing is regulated to 750-900 deg.C and also inequalities are satisfied, by which the internal friction value Q<-1>max of the steel is regulated to (0.5 to 2.0)X10<-4>. By this method, the ultralow carbon steel in which the high formability of IF steel is maintained and deformation and deterioration with the lapse of time at ordinary temp. are prevented can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-low carbon thin steel sheet excellent in workability and formability, which is used for automobiles and the like, overcoming a fatal defect of existing materials, and a method for producing the same.

[0002]

2. Description of the Related Art The so-called ultra-low carbon thin steel sheet is dramatically increasing in consideration of the development of hot rolling technology, cold rolling technology, especially continuous annealing technology, based on the development of steel manufacturing technology. That is, steel that has been completely depleted by vacuum degassing after scouring in steelmaking and further reducing the trace amounts of carbon and nitrogen by adding carbonitride forming elements such as Ti and Nb, which have a strong affinity with them. Therefore, it is also called IF steel (interstitial free steel). The technology described in JP-B-44-18066 is the first. This cold rolled steel sheet has a very high Rankford value (average r value), work hardening index or elongation (El), is excellent in formability and is widely used for automobiles and the like. It was Further, even in the case of hot-rolled steel sheet, it is difficult to increase the Rankford value, but the work hardening index is still high and the formability is excellent, so that the usage amount is increasing also in this field.

Recently, the characteristics have been further improved by further purifying or pursuing hot rolling conditions and continuous annealing conditions. There are many documents on this ultra-low carbon thin steel sheet. As a recent invention relating to this, JP-A-3-1706
The technique described in Japanese Patent No. 18 can be cited as an example.
Further, as an example of the improvement in use performance, with respect to spot weldability, the techniques described in JP-A-63-317647 and JP-A-63-317649 can be cited as examples. Thus, ultra-low carbon steel is being widely used while improving and improving.

[0004]

However, according to the research by the present inventor, a great defect was found in this ultra low carbon steel. It is a change in shape with time at room temperature, which is also called creep at room temperature. After being molded into automobile panels, the shape changes after several months or years. This is because the freezing of the shape becomes a big problem for life automobiles. FIG. 1 is a situation diagram of an experiment to show this.

First, a panel 1 (540 mm × 740 mm) molded in a kamaboko shape (pressing or wrinkling so that the molding shapes are all the same or changing the lubricating conditions) is placed on the support base 2 and moved up and down. A pressure reducing device 5 having an indenter 4 at the tip of a movably attached pressing shaft 3 applies a load of 40 N with the indenter 4 positioned at the center of the panel 1, and then 50
Hold for 0 and 2000 hours. The indenter is made of steel and the tip is a sphere with a diameter of 50 mm. After holding, the load was removed and the curvature κ of the panel was measured. The test steels are typical IF steel and ordinary low carbon aluminum killed steel (JIS G 3141).
SPCE grade), both are commercial steel with a plate thickness of 0.7 mm. The number of repetitions of the experiment was 3. The results are shown in Table 1. The results are expressed by the change in curvature. That is, it is shown by the difference from the curvature κ ₀ measured by unloading immediately after press fitting.

[0006]

[Table 1]

It is known from a separate experiment that, under such a pushing load, the deformation of the steel sheet is in the elastic region.
As shown in Table 1, it is clear that the IF steel is deformed with time despite such a low load.

The present invention aims to overcome this newly discovered defect of IF steel while maintaining the high workability of IF steel.

[0009]

The present inventors have found that this phenomenon is
It is thought that the cause is that dislocations introduced by pressing etc. move easily because F steel has no solid solution carbon and nitrogen.
It succeeded in leaving a very small amount of interstitial solid solution element within the range that does not greatly affect the workability.

That is, the main points of the present invention are:
(1) C ≤ 0.02%, Mn ≤ 0.3
%, S ≦ 0.010%, Al: 0.01 to
0.1%, N ≦ 0.0030%, Ti: 0.
01-0.08%, Nb: 0.005-0.08%, either or both, with the balance being Fe and inevitable impurity elements, and having an internal friction value Q ^-1 max. Of 0.5.
To 2.0 × 10 ⁻⁴ ultra low carbon steel without deformation at room temperature, (2) C ≦ 0.02%,
Mn ≦ 0.3%, S ≦ 0.010%,
Al: 0.01 to 0.1%, N ≤ 0.0030%,
B: 0.0001 to 0.0015% is contained,
Ti: 0.01 to 0.08%, Nb: 0.005 to
A steel containing either or both of 0.08% and the balance being Fe and inevitable impurity elements, and having an internal friction value Q ^-1.
ultra low carbon steel without deformation at room temperature, characterized in that max. is 0.5 to 2.0 × 10 ⁻⁴ , (3) C ≦ 0.02
%, Mn ≦ 0.3%, S ≦ 0.010
%, Al: 0.01 to 0.1%, N ≦ 0.
0030%, Ti: 0.01-0.08%,
Nb: A steel containing either or both of 0.005 to 0.08% and the balance consisting of unavoidable impurity elements is hot-rolled and cold-rolled by a conventional method, and then an annealing temperature ST in continuous annealing.
(° C) is in the range of 750 to 900 ° C and is annealed so as to satisfy the following formula:

[Equation 3] (4) C ≤ 0.02%, Mn ≤ 0.3
%, S ≦ 0.010%, Al: 0.01 to
0.1%, N 2 ≦ 0.0030%, B: 0.
0001-0.0015%, Ti: 0.01-
Steel containing 0.08%, Nb: 0.005 to 0.08%, or both, and the balance of which is an unavoidable impurity element is hot-rolled and cold-rolled by an ordinary method, and then annealed in continuous annealing. A method for producing an ultra-low carbon steel without deformation at room temperature, characterized by performing annealing so that the temperature ST (° C) is in the range of 750 to 900 ° C and satisfying the following formula:

[Equation 4] It is in.

[0011]

The function of the present invention and its operation will be described below. Since C is IF, it must be lowered to an extremely low level. Therefore, it is set to 0.02% or less. It is preferably 0.0050% or less in order to secure workability, and 0.0025% or less in order to aim at the level of superformability.
Mn is 0.3% or less for improving workability. S is an impurity that acts as an inclusion and deteriorates the properties of steel. Therefore, it is set to 0.010% or less.

Al is necessary for deoxidation, but if it is too much, it increases the inclusions and deteriorates the workability of steel. For this reason,
The range was 0.01 to 0.10. N is also an impurity, and if it remains in steel as a solid solution, it causes strain aging deterioration, and if it is fixed with Ti or the like, it is necessary to increase the Ti amount, which unnecessarily increases the price. Therefore, it is set to 0.0030% or less.

To fix the interstitial solid solution element to some extent, T
i and / or Nb are added. The addition amount is in the normal range Ti: 0.01 to 0.08%, Nb: 0.005
It is set to 0.08%. If it is less than the lower limit, the fixing effect cannot be exhibited, and if it exceeds the upper limit, the steel is unnecessarily expensive. IF steel has poor grain boundary strength. Therefore, secondary processing brittleness may occur. When the prevention is required, B is added. The addition amount is 0.0001 to 0.0
015%. If it is less than the lower limit, it has no effect, and if it exceeds the upper limit, not only the effect is saturated, but also the workability is deteriorated.

In the present invention, as another important requirement, it is necessary to control the internal friction value Q ^-1 max. Within the range of 0.5 to 2.0 × 10 ^-4 . Here, the internal friction value Q ^-1 max. Is a snake peak value corresponding to a small amount of solute carbon and nitrogen, and is a peak value that appears near 40 ° C at a frequency of about 1 Hz (from this peak value to the background Here, the internal friction was measured in the torsion mode. The temperature was changed from −40 ° C. to 80 ° C. and the snake peak value was read.

FIG. 2 shows the results of examining the time-dependent deformation in the same manner as in FIG. 1 using steel sheets having various Q ^-1 max. The holding time is 500 hours. As is clear from the figure, by increasing Q ^-1 max., It is possible to prevent this deterioration with time at room temperature as much as with aluminum-killed steel. From this figure, Q ^-1
The range of max. was set to 0.5 × 10 ⁻⁴ or more. The upper limit is 2.0
× 10 ⁻⁴ . This is because if it is more than this, the room temperature non-weatherability, which is one of the features of aluminum-killed steel, cannot be maintained. In addition, so-called BH (bake hardening steel) that secures a large amount of solute carbon and exerts its aging property in this way
(For example, the technique described in Japanese Patent Publication No. 60-47328)
However, the amount of solute carbon is fundamentally different between the BH steel and the present invention steel, and as a result, the present steel is non-aging at room temperature, whereas the BH steel is a delayed aging steel at most. .

Next, as a result of various examinations and examinations of manufacturing conditions for obtaining such conditions, it has been found that the following relationship should be satisfied in the relationship between the components and the final annealing temperature. That is, first, the annealing temperature ST is set to a relatively high temperature of 750 to 900 ° C. If it is less than 750 ° C, it is difficult to secure the Q ^-1 max., And if annealing is performed at more than 900 ° C, a γ phase is partly produced and the material is deteriorated. And an important condition is to satisfy the following relationship between the component χ and the annealing temperature ST.

[Equation 5] If this relationship is not satisfied, the required solid solution carbon, in other words, Q ^-1 max. Cannot be secured.

[0017]

EXAMPLES The present invention will now be described with reference to examples. The steel shown in Table 2 was tapped in a converter. Both are ultra-low carbon steel,
Decarburization in steelmaking was performed by RH vacuum degassing. This steel was hot-rolled and cold-rolled by a conventional method, and then continuously annealed. Hot rolling,
The main conditions for cold rolling are hot rolling finish temperature: 900 to 940.
C, hot rolling coiling temperature: 720-760 C, hot rolled sheet thickness:
The thickness is 3.5 mm and the thickness of the cold rolled sheet is 0.7 mm (cold rolling rate: 80%). T in Table 2! Is the same as the required annealing temperature ST (° C.). Each steel was annealed at the annealing temperature shown in Table 3, then sampled and subjected to various tests.

As is clear from Table 3, those having a component and an internal friction value which are in accordance with the present invention have a small Δκ and are excellent in shape fixability, and the usual mechanical test values are low YP, high El and high rank Ford values. Such high workability is maintained. On the other hand, those which deviate from the present invention have deteriorated shape fixability or poor workability.

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, it is possible to overcome the defects of the ultra low carbon thin steel sheet which is widely used for automobiles and the like due to its excellent formability, and this steel sheet is stable. It can be said that it has built a good foothold as a structural material.

[Brief description of drawings]

FIG. 1 shows a device and method for investigating shape fixability.

FIG. 2 is a diagram showing a relationship between an internal friction value of a steel plate and a shape fixability of a panel.

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[Procedure amendment]

[Submission date] July 8, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

[0006]

[Table 1]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0015

[Correction method] Change

[Correction content]

FIG. 2 shows the results of examining the time-dependent deformation in the same manner as in FIG. 1 using steel sheets having various Q ^-1 max. The holding time is 500 hours. As is clear from the figure, by increasing Q ^-1 max., It is possible to prevent this deterioration with time at room temperature as much as with aluminum-killed steel. From this figure, Q ^-1
The range of max. was set to 0.5 × 10 ⁻⁴ or more. The upper limit is 2.0
× 10 ⁻⁴ . This is because if it is more than this, the room temperature non-aging property , which is one of the features of aluminum killed steel, cannot be maintained. In addition, so-called BH (bake hardening steel) that secures a large amount of solute carbon and exerts its aging property in this way
(For example, the technique described in Japanese Patent Publication No. 60-47328)
However, the amount of solute carbon is fundamentally different between the BH steel and the steel of the present invention, and as a result, the present steel is non-aging at room temperature, whereas the BH steel is a delayed-aging steel at best. .

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0018

[Correction method] Change

[Correction content]

As is clear from Table 3, those having a component and an internal friction value which are in accordance with the present invention have a small Δκ and are excellent in shape fixability, and the usual mechanical test values are low YP, high El and high rank Ford values. Such high workability is maintained. This
On the other hand, those which deviate from the present invention have deteriorated shape fixability or poor workability.

Claims (4)

[Claims] 1. A mass ratio (hereinafter, the same applies to steel components), C ≦ 0.02%, Mn ≦ 0.3%, S ≦ 0.010%, Al: 0.01 to 0.1%, N ≦ 0.0030%, Ti: 0.01 to 0.08%, Nb: 0.005 to 0.08%, or both, with the balance being Fe and an unavoidable impurity element. An ultra-low carbon steel that does not deform at room temperature and is characterized by having an internal friction value Q ^-1 max. Of 0.5 to 2.0 × 10 ^-4 . 2. C ≦ 0.02%, Mn ≦ 0.3%, S ≦ 0.010%, Al: 0.01 to 0.1%, N ≦ 0.0030%, B: 0.0001 to 0.0015%, Ti: 0.01 to 0.08%, Nb: 0.005 to 0.08%, or both, with the balance being Fe and inevitable impurity elements. , An internal friction value Q ^-1 max. Is 0.5 to 2.0 × 10 ^-4, which is an ultra low carbon steel which is not deformed at room temperature. 3. C: 0.02%, Mn ≤ 0.3%, S ≤ 0.010%, Al: 0.01 to 0.1%, N ≤ 0.0030%, Ti: Steel containing 0.01 to 0.08%, Nb: 0.005 to 0.08%, or both, and the balance of which is an unavoidable impurity element is hot-rolled and cold-rolled by a conventional method, Annealing temperature ST in continuous annealing
A method for producing an ultra-low carbon steel that does not deform at room temperature with time, characterized in that (C) is in the range of 750 to 900 ° C. and is annealed so as to satisfy the following formula. [Equation 1] 4. C ≦ 0.02%, Mn ≦ 0.3%, S ≦ 0.010%, Al: 0.01 to 0.1%, N ≦ 0.0030%, B: 0.0001 to 0.0015%, Ti: 0.01 to 0.08%, Nb: 0.005 to 0.08%, or both, with the balance being an unavoidable impurity element Annealing temperature ST in continuous annealing after hot rolling and cold rolling at
A method for producing an ultra-low carbon steel that does not deform at room temperature with time, characterized in that (C) is in the range of 750 to 900 ° C. and is annealed so as to satisfy the following formula. [Equation 2]