JPH11192502A - Tailored steel strip for press forming and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a tailored steel strip whose thickness is different or the thickness and strength are different in the width direction of the steel strip without executing shearing in the width direction of the steel strip and welding in the longitudinal direction of the steel strip, and manufacturing method of it. SOLUTION: In finish rolling of continuous hot rolling, using rolling work rolls having differential thickness and/or taper, the material is rolled into a shape having different thickness in the width direction of the steel strip and, next, in water-cooling on a run out table after finish rolling of the continuous hot rolling, by changing cooling rate in the width direction of the steel strip, the steel strip different in mechanical properties such as tensile strength, yield strength, elongation and deep drawability is made.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tailored steel strip for press forming in which the thickness of the steel strip is varied in the width direction without shearing the steel strip in the width direction and welding the steel strip in the longitudinal direction. The present invention relates to a tailored steel strip having different thickness and mechanical properties, and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, with the progress of laser welding and mash seam welding, steel strips having different strengths and / or different plate thicknesses have been welded and integrated as a blank material for a body part of an automobile or the like. Steel strips are being manufactured for tailored blanks. When this steel strip is pressed into a body part, it is possible to make the mechanical properties and / or the plate thickness of a predetermined part of the body part different.

A conventional method and product for manufacturing a tailored steel strip will be described with reference to the drawings.

FIG. 1 is a diagram showing an outline of a manufacturing process of a tailored steel strip. As shown in FIG. 1, a method for manufacturing a tailored steel strip includes, for example, a method of winding a coil edge while rewinding three sheet thicknesses or steel strips A, B, and C having different thicknesses and strengths, which have been sheared to a predetermined width. Is welded in a laser welding apparatus equipped with a welding torch 1 so as to have a joining line in the longitudinal direction of the steel strip to form a steel strip for press forming. Coil. This process is commonly referred to as tailored blanking or tailored blank welding. FIG. 2 is a diagram showing an example of a center pillar of an automobile manufactured by press-forming a steel strip joined in the width direction by this process. As shown in FIG. 2, the center pillar 3 of the press-formed automobile is formed by welding steel strips A, B, and C at a joining portion 4, and has a thick central portion and a high strength state. .
That is, the steel strip A and the steel strip C have a thickness of 1 mm and a pressure of 300 MPa.
Is the yield strength. Steel strip B is 2mm thick and 600MP
This is an example in which the yield strength is a.

[0005] Known examples of such tailored blanks include, for example, JP-A-8-281306, JP-A-7-290182, and JP-A-8-174.
No. 246 has been proposed.

In each of these known examples, a steel strip having a different thickness is sheared in the width direction, and then, the sheared steel strips having a predetermined thickness are combined and joined in the width direction to form a laser weld or a mash seam. Joining is performed by welding. For this reason, even if sound welding has no weld defects, a difference in hardness between the weld and the base material and / or a weld undercut occurs, so that stress concentration of press working occurs in the weld during pressing, May crack. Furthermore, if there is a welding defect even in the part, a crack occurs at the time of pressing, and there is a problem that the press forming line must be stopped. In addition, welding of steel strips of different thicknesses and different strengths is performed by current welding techniques such as laser welding, mash seam welding, arc welding or high-frequency welding. There are many problems in terms of stability, and there is a problem that welding defects are likely to occur.

Further, these conventional methods have a large tailored blank effect, but have a problem that the tailored blank effect is almost halved as a whole because of the high shearing cost and welding cost.

[0008]

SUMMARY OF THE INVENTION In order to eliminate the above-mentioned problems, the present invention has a different thickness or a different thickness, such as a pillar-like part such as a frame structure of an automobile or a floor, or a floor. Tailored steel strip for press forming suitable for manufacturing parts that require different strengths is made into a tailored steel strip for press forming without welding as before, and low cost, productivity and It is an object of the present invention to provide a method for manufacturing the tailored steel strip for press forming in which the yield is dramatically improved.

[0009]

According to the present invention, in order to solve the above-mentioned problems, a rolled work roll having a special shape is used as a finish rolling roll, and steel strips having different thicknesses in the width direction are continuously hot-rolled. Can be manufactured. Further, in the cooling step after finish rolling of continuous hot rolling, a cooling method is devised so that steel strips having different mechanical properties in the width direction can be manufactured.

The gist of the present invention is as follows.

(1) A tailored steel strip for press forming, characterized in that two or more steel strips having different sheet thicknesses are formed integrally by rolling in the width direction of the steel strip.

(2) A tailored steel strip for press forming, wherein two or more steel strips having different thicknesses and mechanical properties are integrally formed by rolling in the width direction of the steel strip.

(3) In the finish rolling of continuous hot rolling, a steel strip is rolled by using a rolled work roll having a difference in thickness and / or a taper, and two or more kinds of sheet thickness differences in the width direction of the steel strip. The method for producing a tailored steel strip for press forming according to the above (1), wherein the steel strip has a bent shape.

(4) In the finish rolling of the continuous hot rolling, using a rolled work roll having a difference in thickness and / or a taper, the steel strip is rolled into a shape having a different thickness in the width direction of the steel strip. In water cooling on the run-out table after finish rolling of the steel strip, by changing the cooling rate in the width direction of the steel strip, mechanical strength such as tensile strength, yield strength, elongation, deep drawability, etc. The method for producing a tailored steel strip for press forming according to the above (2), wherein the steel strips have different properties.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings.

FIG. 3 is a view showing an example of a final roll of finish rolling of continuous hot rolling in the present invention. FIG. 4 is a view showing a steel strip rolled by the final roll of FIG. FIG. 4 is a view showing a center pillar of an automobile obtained by press forming the steel strip of FIG. 4 rolled by the final roll of FIG. 3.

The roll 5 shown in FIG. 3 has a concave central portion. When the roll 5 is used as an upper work roll and a steel sheet is rolled, as shown in FIG. , E and F are different from each other, and a steel strip having a plate thickness E at the center portion larger than plate thicknesses D and F at the side portions can be manufactured. The shape of this final roll may be concave, only the WS (rolled work side) portion may be convex or concave, or only the DS (rolled drive side) portion may be convex or concave. Further, the shape may be a combination of convex and concave. By using such a roll, two or more types of tailored steel strips having different thicknesses in the width direction of the steel strip can be manufactured.
However, if rolling is carried out with a normal roll after this rolling, the projections and depressions that have been attached will be lost, and therefore it is desirable to carry out rolling with the final roll of finish rolling. However, for example, since there is no rolling power of the final rolling roll, rolling is performed by applying a special roll having the shape described above with the preceding roll, and the final normal roll is not rolled, but is about a normal correction. Leveling may be performed.

The D and F portions of the steel strip 6 shown in FIG.
m, and the thickness of the portion E is 4 mm. As a result of performing a tensile test on this steel strip and measuring the yield strength for each of the D portion, the E portion, and the F portion, the D portion was 305 MPa, the E portion was 302 MPa, and the D portion was 306 MPa.

The d of the center pillar 3 of the vehicle shown in FIG.
The thickness of the part and the part f is 2 mm, while the thickness of the part e is 4 m
m. This not only makes it possible to produce the same center pillar as a tailored blank by conventional laser welding, but also because there is no laser welding, welding costs can be omitted, welding defect inspection steps can be omitted, and welding defects at the time of pressing Can be completely eliminated. Of course, the present invention is also extremely excellent in the yield improvement and the productivity of the steel material, which are the original objects of the conventional tailored blank.

Next, a cooling method for changing the mechanical properties of the steel strip in the width direction will be described.

FIG. 6 is a diagram showing an example of a water cooling method from the upper surface and / or lower surface of the run-out table after finish rolling of continuous hot rolling. In this example shown in FIG. 6, only the central portion (H portion) in the width direction of the steel strip 6 is quenched by the water cooling nozzle 7 from the upper surface and the lower surface, while the G portion and the I portion in the width direction of the steel strip 6 are: The gradual cooling is performed without performing the rapid cooling by the water cooling nozzle 7 from the upper surface and the lower image. Further, for slow cooling, water cooling may be performed by covering the G portion and the I portion with a steel plate or the like.

The mechanical properties of the steel strip 6 cooled on the run-out table after the finish rolling of the continuous hot rolling were investigated. That is, as a result of performing a tensile test at the position in the width direction of the steel strip 6, the yield strength of the G section was 306 MPa, and the yield strength of the H section was 6 MPa.
It was 10 MPa, and the yield strength of Part I was 304 MPa. The steel strip can be press-formed to produce a center biller for an automobile. Further, by changing the cooling method, it is also possible to control the mechanical properties other than the yield strength, such as workability such as deep drawability and fatigue strength, in the width direction of the steel strip. Furthermore, instead of manufacturing by continuous hot rolling, it can also be manufactured by coating a required portion of a steel strip after cooling with a heat-resistant material or the like, and then heating and cooling with water.

As described above, according to the present invention, not only can the same center pillar as the tailored blank formed by conventional laser welding or the like be manufactured, but also since there is no laser welding, the welding cost can be reduced, and the inspection process for welding defects can be omitted. It is possible to eliminate press cracks caused by welding defects at the time of pressing. Of course, the present invention is also extremely excellent in the yield improvement and the productivity of the steel material, which are the original objects of the conventional tailored blank.

[0024]

EXAMPLES Specific examples of the present invention will be described below.

The chemical composition of the steel of the tailored steel strip of the present invention,
Table 1 shows the rolling conditions, the shape of the obtained steel strip, and the results of the tensile test (mechanical properties).

As shown in Table 1, in this example of the present invention, 12
The width direction of the steel strip having a width of 00 mm is equally divided into three sections A, B, and C, and the thickness of the steel strip at the center (width B) is 3.2 m.
m, using a roll with a concave central portion as an upper work roll so that the thickness of the other portions (A and C portions) becomes 1.6 mm, finish rolling by continuous hot rolling, and tailoring the steel strip. Manufactured. Example No. 1 and No. No. 2 was used to cool the steel strips A, B and C at the same cooling rate when water-cooling them with a run-out table after finish rolling. Example No. 3
When water cooling at the run-out table after finish rolling,
The cooling rate of the part B was 23 ° C./sec, which was faster than that of the other parts A and B at 7 ° C./sec. A, B of the obtained steel strip
And each of the test pieces was prepared from part C, and a tensile test was performed. As a result, Example No. 1 and No. In 2, A,
The mechanical properties (YS, TS and El) of the parts B and C had almost the same value.

However, in the case of No. In No. 3, the width central portion (B portion) was thick, and the yield strength and tensile strength were significantly higher than those of the A and C portions. On the other hand, the elongation at the width center portion (B portion) was lower than that at the A and C portions.

As apparent from the above test results, according to the present invention, a tailored steel strip for press forming, in which two or more steel strips having different thicknesses in the width direction of the steel strip are integrally formed by rolling, Alternatively, a tailored steel strip for press forming is obtained in which two or more types of steel strips having different thicknesses and mechanical properties are integrally formed by rolling in the width direction of the steel strip. It was confirmed that the material was suitable for a tailored blank in view of its mechanical properties.

[0029]

[Table 1]

[0030]

According to the present invention, the conventional tailored blank welding process is omitted, so that the welding cost can be reduced while improving the yield and the productivity of the steel material, which is the object of the conventional tailored blank, while maintaining excellent welding. Inspection process can be omitted and press cracks caused by welding defects at the time of press can be completely eliminated, dramatically increasing the productivity and manufacturing cost of press forming of automobiles, building machines, industrial machines, etc. This has the effect of being able to be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of a conventional tailored blank system or tailored blank welding.

FIG. 2 is a diagram showing an example in which a tailored blank steel strip manufactured by the process of FIG. 1 is press-formed into a body part.

FIG. 3 is a view showing an example of a final finishing roll of the present invention.

FIG. 4 is a view showing an example of a steel strip rolled by the roll of FIG. 3;

FIG. 5 is a view showing an example of a vehicle body part obtained by press-forming the steel strip of FIG. 4;

FIG. 6 is a diagram showing an outline of a cooling method in a run-out table.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Welding torch 2 Joined steel strip for press 3 Center pillar 4 Joining part 5 Roll 6 Steel strip 7 Water cooling nozzle A Steel strip B Steel strip C Steel strip

Claims (4)

[Claims] 1. A tailored steel strip for press forming, wherein two or more types of steel strips having different thicknesses in the width direction of the steel strip are integrally formed by rolling. 2. A tailored steel strip for press forming, wherein two or more steel strips having different thicknesses and mechanical properties are integrally formed by rolling in the width direction of the steel strip. 3. In the finish rolling of continuous hot rolling, a steel strip is rolled by using a rolled work roll having a difference in thickness and / or a taper, and two or more kinds of sheets having different thicknesses in the width direction of the steel strip. The method for producing a tailored steel strip for press forming according to claim 1, wherein the steel strip has a shape. 4. In the finish rolling of the continuous hot rolling, the steel strip is rolled to a shape having a different thickness in the width direction of the steel strip using a rolled work roll having a difference in thickness and / or a taper. In water cooling on a run-out table after finish rolling, the mechanical properties such as tensile strength, yield strength, elongation, deep drawability, etc. in the width direction of the steel strip by changing the cooling rate in the width direction of the steel strip 3. The method for producing a tailored steel strip for press forming according to claim 2, wherein the steel strip is a different steel strip.