JPH0573820B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to a cold-rolled steel sheet having non-uniform strength in the width direction and a method for manufacturing the same, and particularly to a cold-rolled steel sheet having non-uniform strength in the width direction, which is suitable for automobile outer panels, etc. It relates to a cold-rolled steel sheet with strength and a method for manufacturing the same.

[Background of the Invention] In order to reduce the weight of automobiles, steel plates used are being made thinner. However, there is a limit to thinning the parts in order to ensure their tension and rigidity, and careful consideration is required to further reduce weight. In general, the central part of large-sized exterior parts such as automobiles has a small curvature, so the tension stiffness is low, but the closer you get to the edges, the greater the curvature, and the outer periphery is fixed or reinforced in some way, so the tension is low. High rigidity.

Conventionally, for such parts, the 13th
A large crown steel plate with a drum-shaped cross-section as shown in the figure was used.

However, such steel sheets require special minus-crown rolls in all processes such as rolling and skin-passing, and when rolled in large quantities with large minus-crown rolls, there is a risk of roll breakage accidents.

On the other hand, efforts have also been made to meet the above requirements from the viewpoint of materials such as high-strength steel whose composition has been adjusted.
However, when high-strength steel is used, various elements (C, Si, Mn, P, etc.) are added to increase the strength, which inevitably reduces workability.

[Purpose of the invention] The present invention does not require composition adjustment or special rolling rolls, and can ensure workability on the edge side of a steel plate, which is subject to severe processing, and also ensure strength at the center of the width, where strength is required. It is an object of the present invention to provide a cold rolled steel sheet having non-uniform strength in the width direction and a method for manufacturing the same.

[Summary of the Invention] The first invention according to the present application is a cold rolled steel plate having a substantially uniform thickness in the width direction and manufactured through a hot rolling process, a cold rolling process and a continuous annealing process, The area near the edge in the width direction has deep drawability with an r value of 1.5 or more, and the area near the center in the width direction has a drawability of 35 to 60 kg/
mm ² , while the area near the edge has a high tensile strength of 28 to 40 mm.
By staying at a tensile strength of Kg/ ^mm2 , the tensile strength near the center in the width direction and near the edges is 4Kg/ ^mm2.
A cold rolled steel sheet having non-uniform strength in the width direction, characterized by having the above characteristics.

Examples of steel sheets to which the present invention can be applied include panel applications such as automobile roofs, bonnets, desk tops, and cabinets.

Here, the substantially uniform plate thickness is a plate thickness having a general crown of about 20 to 80 μm.

A second invention according to the present application is a method for manufacturing a cold rolled steel sheet including a hot rolling step, a cold rolling step and a continuous annealing step, in which the edge in the width direction of the steel sheet is removed after finish rolling in the hot rolling step. The steel plate is rolled up while maintaining the central part in the width direction at a temperature of 650°C or higher and the center part in the width direction at a temperature of 600°C or lower.
A cold rolled steel plate having non-uniform strength in the width direction, which is characterized by cold rolling at a rolling reduction of 85%, and then heating the steel plate to a temperature of 700 to 850°C in a continuous annealing process to recover and recrystallize it. This is a manufacturing method.

In the present invention, since the coiling is performed at a low temperature of 600° C. or lower near the center, the carbides become fine and are re-dissolved during continuous annealing. Therefore, the amount of solid solution [C] increases and the part becomes hard. on the other hand,
Since the area near the edge is wound at a high temperature of 650° C. or higher, the carbide does not re-dissolve into solid solution, the amount of solid solution [C] decreases, and the area becomes soft.

In the present invention, in order to maintain the temperature of the central portion in the width direction at 600° C. or lower, for example, cooling water or the like may be jetted into the central portion.

Note that the finish rolling finishing temperature may be about 850 to 930°C as long as it is higher than the transformation point.

When the temperature of the edge part is less than 650℃, cementite does not form into lumps and is easily re-dissolved in the subsequent continuous annealing process, and even after continuous firing, the solid solution [C] increases, resulting in softening of the edge part. I can't get it.

A third invention according to the present application is a method for manufacturing a cold rolled steel sheet including a hot rolling process, a cold rolling process, and a continuous annealing process, in which the continuous annealing process is performed after the hot rolling process and after the cold rolling process. , the edge part of the steel plate
This method of manufacturing a cold-rolled steel sheet having non-uniform strength in the width direction is characterized in that the steel sheet is heated to 600 to 850° C., while the central portion of the steel sheet is heated at a temperature below the recrystallization temperature of the steel.

In the present invention, since the edge portion is heated to a high temperature of 600 to 850° C. in the continuous annealing process, recovery and recrystallization occur in this portion, and the crystal grains become equiaxed grains. Therefore, that part becomes soft. on the other hand,
Since the central part is annealed at a temperature below the recrystallization temperature, only recovery occurs, the processed structure remains, and that part becomes hard.

Note that in order to maintain the width direction edge portion at a temperature of 600 to 850° C., for example, an induction heating device or a radial tube movable in the width direction may be used.

[Example of the invention] (Example of the second invention) A steel material having the following components was rolled to 2.6 mm in a hot rolling process.
Made of thick steel plate.

Ingredients C: 0.03, Si: 0.01, Mn: 0.20, P:
0.013, S: 0.012, Al: 0.042, remaining Fe.

The finishing temperature during hot rolling was 870°C.

After finish rolling, the winding temperature was adjusted using the apparatus shown in FIG.

That is, the cooling water bedders 2 and 6 each having a nozzle 3 are arranged above and below the steel plate 8, and the steel plate 8 is
A shielding plate 4 is disposed to cover the edge portion so that cooling water does not hit the edge portion. Cooling water main pipes 1 and 5 are connected to cooling water headers 2 and 6.
is connected.

For steel plate 8 that finished finish rolling at 870℃,
Cooling water was spouted from the cooling water bedders 2 and 6, and the winding temperature was 710°C at the edges and 600°C at the center. The temperature distribution at that time is shown in Figure 2.

Note that by changing the amount of shielding plate 4 that covers steel plate 8, the width of the central portion at 600° C. can be increased or decreased as appropriate.

The steel plate was then cold rolled from 26mm to 0.8mm.

After cold rolling, it was heated at 800°C for 60 seconds, rapidly cooled to 400°C, and then overaged at 400°C for 3 minutes.

When the mechanical strength of the steel plate thus manufactured was measured, the results shown in FIGS. 3 and 4 were obtained. That is, the tensile strength was 33 to 35 Kg/mm ² near the edges, and 37 Kg/mm ² or more near the center. Furthermore, the elongation was 40 to 50% near the edges and about 30% near the center.

Further, when a workability test was conducted, the γ value of the edge portion was 1.55 or more.

As a comparative example, the case where the winding temperature was 710° C. over the entire width is shown by dotted lines in FIGS. 2 to 4. The strength and elongation of this comparative example are almost the same as those of the central part of this example.

Therefore, according to the steel plate of this example, when processed into a shape like the roof of a car as shown in FIG. 5, the edges are soft and can be processed sufficiently, while the central part is hard and can be processed easily. ,or,
As shown in Fig. 6, it has sufficient strength for the shape of steel furniture, switchboards, etc., so it is possible to reduce the plate thickness.

(Test example of the second invention) For steel having the following components, the coiling temperature was 550°C.
The temperature was changed to ~750°C, and the tensile strength and elongation were measured.

Ingredients C: 0.04, Si: 0.01, Mn: 0.20, P:
0.015, S: 0.010, Al: 0.04.

Note that continuous annealing was performed at 700°C for 60 seconds over the entire width.
The results are shown in FIG. As can be seen from FIG. 11, if the temperature of the edge portion is lower than 650° C., the tensile strength of this portion becomes high and the desired workability cannot be obtained. Furthermore, if the temperature in the center exceeds 600°C, that part becomes soft and the desired strength cannot be obtained.

(Example of the third invention) A steel material having the following components was heated to 2.6 mm by hot rolling process.
Made of thick steel plate.

Ingredients C: 0.003, Si: 0.01, Mn: 0.23, P:
0.013, S: 0.010, Al: 0.038, Ti: 0.074, remaining Fe.

The finishing temperature during hot rolling was 900°C, and the coiling temperature was 680°C.

The steel plate was then cold rolled from 26mm to 0.8mm.

After cold rolling, continuous annealing was performed as shown in FIG. That is, the edge portion is heated using the radial and tube 13 movable in the width direction or the induction heating device 14, and the edge portion is heated to 820°C (the recrystallization temperature of this steel plate is approximately 720°C) and the center The sample was brought to a temperature below 700°C and held for 60 seconds. The temperature distribution in the width direction of the steel plate at that time is shown in FIG.

Note that by appropriately moving the radiand tube in the width direction, the width direction temperature range of 820° C. can be changed as appropriate.

Thereafter, it was rapidly cooled to 400°C, and then over-aged at 400°C for 3 minutes.

When the distribution of mechanical properties in the width direction of the steel plate thus produced was measured, the results were obtained as shown in FIG. 3 for tensile strength and FIG. 4 for elongation at break. That is, the tensile strength was 30 to 32 Kg/mm ² near the edges, and 40 Kg/mm ² or more near the center. Furthermore, the elongation was 40 to 50% near the edges and about 25% near the center.

When this steel plate was tested for workability, the γ value of the edge portion was 2.0 or more.

As a comparative example, the total width was measured at 60°C at 830°C.
8 to 1 for the case of continuous annealing for seconds.
It is shown by the dotted line in Figure 0. The strength and elongation of this comparative example are almost the same as those of the central part of this example.

Therefore, according to the steel plate according to this embodiment, similar to the first embodiment, it can be used in a shape like the roof of a car as shown in FIG. 5, steel furniture, a power distribution board, etc. as shown in FIG. When the shape of the sheet is machined, the edges are soft and can be machined sufficiently, while the central part is hard and has sufficient strength as a structure, making it possible to reduce the plate thickness.

(Test Example of the Third Invention) The tensile strength and elongation of steel having the same composition as used in the test example of the second invention were measured by changing the continuous annealing temperature.

Note that the hot rolling coiling temperature was 710°C for the entire width.

The results are shown in FIG. As can be seen from Figure 12, if the temperature is less than 600℃, the tensile strength will be too high and the desired workability cannot be obtained, and if the temperature exceeds 650℃,
The elongation increases and the desired strength cannot be obtained.

[Effects of the Invention] The first invention of the present application is that the edge side is made of mild steel material (ultra deep drawing material with composition γ = 2.0), and the center part is made of a material having a tensile strength of 35 to 60 Kg/mm ² . Therefore, there is no need for component adjustment or special rolling rolls, and by simply controlling the temperature in the width direction, we can ensure workability on the edge side of the board, which is subject to severe processing, and also ensure strength in the center of the width, where strength is required. can,
When processed into a shape like the roof of a car,
The edges are soft enough to be machined, while the center is hard enough to be used as a structure.
It is possible to reduce the plate thickness.

Moreover, according to the second invention and the third invention of the present application, the steel plate according to the first invention can be manufactured.

[Brief explanation of drawings]

FIG. 1 is a conceptual side view of the apparatus used in the first embodiment. FIG. 2 is a graph showing the temperature distribution in the width direction of the steel plate in the first example.
FIG. 3 is a graph showing the strength distribution in the width direction of the steel plate according to the first example. FIG. 4 is a graph showing the elongation distribution in the width direction of the steel plate according to the first example. FIG. 5 and FIG. 6 are perspective views showing processing examples of the steel plate of the first invention according to the present application. FIG. 7 is a conceptual front view of the device used in the second embodiment. FIG. 8 is a graph showing the temperature distribution in the width direction of the steel plate in the second example. Figure 9 shows the second
It is a graph showing the strength distribution in the width direction of the steel plate according to the example. FIG. 10 is a graph showing the elongation distribution in the width direction of the steel plate according to the second example. FIG. 11 is a graph showing the relationship between hot winding temperature, tensile temperature, and elongation. FIG. 12 is a graph showing the relationship between continuous annealing temperature, tensile strength, and elongation. FIG. 13 is a front view showing a conventional cold-rolled steel plate.

Claims (1)

[Scope of Claims] 1. A cold-rolled steel plate having a substantially uniform thickness in the width direction manufactured through a hot rolling process, a cold rolling process, and a continuous annealing process, in which r near the edge portion in the width direction It has deep drawability with a value of 1.5 or more, and has a high tensile strength of 35 to 60 Kg/mm ² near the center in the width direction, while maintaining a tensile strength of 28 to 40 Kg/mm ² near the edges. A cold-rolled steel sheet having non-uniform strength in the width direction, characterized in that the tensile strength near the center portion in the width direction and near the edge portions is 4 Kg/mm ² or more. 2. In a method for producing a cold rolled steel sheet including a hot rolling process, a cold rolling process and a continuous annealing process, after finish rolling in the hot rolling process, the edge portion in the width direction of the steel plate is heated to a temperature of 650°C or higher, In addition, the steel plate is rolled up while maintaining the center part in the width direction at a temperature of 600°C or less, and then cold rolled at a rolling ratio of 60 to 85%, and then the steel plate is subjected to a continuous annealing process at a temperature of 700 to 850°C. A method for producing a cold-rolled steel sheet having non-uniform strength in the width direction, the method comprising heating to recover and recrystallize the steel sheet. 3. In a method for producing a cold rolled steel plate including a hot rolling process, a cold rolling process and a continuous annealing process, after the hot rolling process, the steel plate is cold rolled at a rolling reduction of 60 to 85%, and then the continuous annealing process The edge part of the steel plate is 600~
A method for producing a cold-rolled steel sheet having non-uniform strength in the width direction, characterized in that the process is carried out at a temperature of 850° C. at a temperature below the recrystallization temperature of the steel sheet.