JPH03133630A - Clad steel sheet having good formability excellent in dent resistance and surface strain resistance - Google Patents

Abstract

PURPOSE:To obtain steel sheet excellent in workability, dent resistance and surface strain resistance by specifying the amount of the respective components of C, Mn, Al, P, Si, S and N contained in an inner layer and an outer layer. CONSTITUTION:The title clad steel sheet is constituted of a three-layer structure consisting of the high-strength layer of the outer layers 51 and the soft layer of an inner layer 61. The constitutional ratio of thickness of this sheet and strength is optimized. In other words, the component of the inner layer 61 consists of by weight 0.01 - 0.10% C, 0.05 - 1.5% Mn, 0.01 - 0.10% Al, <= 0.15% P, <= 0.4% Si, impurities of <= 0.02% S and <= 0.0080% N and the balance Fe and inevitable impurities. The component of the outer layer 51 consists of by weight 0.01 - 0.15% C, 0.05 - 2.0% Mn, 0.01 - 0.10% Al, <= 0.20 P, <=0.5% Si and impurities of <= 0.02% S and <= 0.0080% N and the balance Fe and inevitable impurities. Total thickness of both outer layers 51 is regulated to 10 - 40% of the whole thickness. Tensile strength of the outer layer 51 is 35 - 55 kgf/mm<2>. Tensile strength of the inner layer 61 is 25 - 45 kgf/mm<2>. The tensile strength of the outer layer 51 is made higher than the tensile strength of the inner layer 61 by at least 10 kgf/mm<2>.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an M-killed clad steel sheet having excellent dent resistance, surface strain resistance, and good formability, and particularly relates to a cold-rolled clad steel sheet suitable for outer panels. This relates to steel plates.

BACKGROUND OF THE INVENTION In recent years, there has been a demand for steel plates with excellent dent resistance from the viewpoint of reducing weight and improving safety. Conventionally, in response to these requests,
This problem has been addressed by increasing the strength of steel plates. However, increasing the strength of the steel sheet is accompanied by a decrease in formability, and furthermore, increasing the strength of the outer plate causes a new problem related to the appearance of the molded product, such as the occurrence of surface distortion due to the increase in yield strength. In fact, the problem of increasing the strength of the rear fender and quarters, which are difficult-to-form exterior parts, remains unsolved.

Tokukai Sho 57-! According to No. 34522, an attempt has been made to solve the above problem by controlling the primary cooling of continuous annealing to form a composite structure only on the surface, but the range of characteristic values of the steel sheet is narrow and the degree of freedom is small.

In addition, conventional methods have been used to harden the surface by surface carburizing or nitriding, but these methods are not only expensive, but also
It is difficult to control the hardening amount and hardening depth of the surface layer of the steel sheet, and furthermore, it can be applied only to final products because plastic working causes cracking.

Problems to be Solved by the Invention The present invention has a three-layer structure consisting of a high-strength outer layer and a soft inner layer, and by optimizing the plate thickness and strength composition ratio, the present invention solves problems that cannot be achieved with ordinary steel plates. This is a steel plate that has a certain level of dent resistance, surface distortion resistance, and good formability.

Means to solve problems The present invention is.

(1) Components of the inner layer in weight%, C: 0.01 to 0.1
0%, Mn: 0-05~1.5%, Al: 0.0
1 to 0.10%, P: 0.15% or less, Si:
0.4% or less, S as impurities is 0.02% or less, N is 0.0080% or less, the balance is Fe and unavoidable impurities, the outer layer component is in weight%, C: 0.01-0
．． 15%, Mn: 0.05-2.0%, AM:
0-01~0.10%, P: 0.20% or less, Si
: Contains 0.5% or less and 0.0 S as an impurity.
2% or less, N is 0.0080% or less, and the balance is Fe and unavoidable impurities, a three-layer structure clad steel plate with good formability and excellent dent resistance and surface strain resistance, and (2) both outer layers. The total thickness of the outer layer is 10 to 40% of the total thickness, and the tensile strength of the outer layer is 35 to 55 kgf/■2. Excellent dent resistance and surface distortion resistance according to claim 1, wherein the inner layer has a tensile strength of 25 to 45 kgf/+*m2, and the outer layer has a tensile strength that is 10 kgf/am2 or more higher than the inner layer tensile strength. It is a clad steel plate with good formability.

Effect: C, Mu, A1. of the present invention. P, Si, S, H
Regarding the reasons for limiting the numerical values for each component, we will first discuss the inner soft layer.

C is an element that increases the strength of a steel plate, but its upper limit is limited by workability. If C exceeds 0.10%, the desired formability cannot be obtained, and the lower limit is limited by secondary processability, and is set to 0.01% or more. Secondary workability refers to the durability of press-formed products against brittle cracking, and those that do not generate brittle cracks at lower temperatures are said to have good secondary workability.

The lower limit of Mn and the upper limit of S are limited from the viewpoint of preventing FeS generation. In other words, when the Mn/S ratio is small, S becomes FeS
This causes hot brittleness. Therefore, Kn must be at least 0.05% and S must be at most 0.020%.

M needs to be 0.01% or more to promote AIIN precipitation, but if it is too large, it will harden the steel and deteriorate the r value, so it needs to be 0.10% or less.

From the viewpoint of workability, N needs to be 0.0080% or less.

P, Mn, and Si are reinforcing elements, but from the viewpoint of workability, the upper limits are 0.15% or less, 1.5% or less, and 0.
．． It needs to be 4% or less.

Next, the reason for limiting the numerical values for each component of the outer layer will be described.

C is an element that increases the strength of the steel plate, but if it is too high, it deteriorates workability, so it needs to be kept at 0.15% or less. The lower limit is limited by secondary processability and is set to 0.01% or more.

P, Mn, and Si are reinforcing elements, but the upper limits are 0.20% or less, 2.0% or less, and 0, respectively, from the viewpoint of workability.
．． It needs to be 5% or less. Lower limit of Mn and M, N
The reason for the limitation is the same as that for the inner layer.

Next, from the viewpoints of dent resistance, surface strain resistance, and formability, we will discuss the reasons for limiting the plate thickness and strength composition ratio.If the total thickness of both outer layers is less than 105, the effect of improving dent resistance will be small.
If it exceeds 40%, the deterioration of workability and surface strain resistance will increase.

The reason why the upper limit of the tensile strength of the surface layer is set to 55 kgf/am2 is that if the surface is made any harder, the formability may be impaired.The lower limit of the tensile strength of the inner layer is set to 25 kgf/am2.
The reason why it is set to mm2 is that it is practically impossible to produce a low carbon aluminum guild steel according to the present invention with a tensile strength of 25 kgf/am2 or less.

The reason why the tensile strength difference between the inner layer and the outer layer is set to 10 kgf/am2 is because the effect of improving dent resistance is small and ineffective if it is less than this.The lower limit of the tensile strength of the outer layer is 35 kgf/am2 due to the limit of 0 or more. ．． The upper limit of the tensile strength of the inner layer is 4
It becomes 5kgf/llm2.

The clad steel of the present invention can be produced by methods generally known for producing clad metal plates, such as lap rolling and ingot casting, but continuous production is possible from the viewpoint of bonding strength between the inner and outer layers and work efficiency. Casting methods are particularly suitable.

Figure 2 shows a continuous casting method, in which 1 is a mold, 2 and 3 are immersion nozzles, each having different lengths.
Molten metals having different compositions are injected into the cast cladding 1. 4 is a molten metal pool, 5 is a surface layer portion of the composite steel material, and 6 is an inner layer solidified portion of the composite steel material. A magnet 7 forms a static magnetic field in the casting direction and in a direction perpendicular to the plane of the paper. This static magnetic field brakes the flow in the molten metal pool caused by injection, minimizes mixing of the upper and lower layers at the positions where they touch, and allows casting of a slab 8 with different compositions in the inner and outer layers.

In the present invention, the slab obtained as described above is subjected to hot rolling, pickling, cold rolling, annealing, and pressure adjustment to produce a steel plate (clad steel plate), but the process after hot rolling is performed using ordinary materials. It is similar to

Next, the present invention will be specifically explained with reference to Examples.

Example 1 As shown in FIG. 1, the total thickness of both outer layers 51 is 20% of the total thickness t (lO% on one side), the tensile strength of the inner layer 61 is limited to 33 kgf/mm2, and the tensile strength of the outer layer is Strength to 33
Varying from kgF/m intestine 2 to eokgf/mm2,
Furthermore, the total thickness t is 0.70 to 0.80 mm (7).
A dent test was conducted using a composite steel material with a diameter of 0.05 mm.

The test conditions are as shown in Figure 3, and span a =
4hmc7) Punch the center with a punch 9 of 10mm radius (R) 3
A load W of 0 kgf/ram2 was applied. Dent resistance was then evaluated based on the amount of dent after unloading. 10 is sample, b=5
01, C=50 layers.

According to the results in Figure 4, the tensile strength of the outer layer steel plate is 45 kg.
In the case of f/m shoes 2, the total thickness is 0.75m■, and when the tensile strength of the outer layer steel plate is 80kgf/mm2, the total thickness is 0.70+W
m, and the tensile strength is 33 kg4/mm2. Total thickness is 0.8
The dent depth is the same as that of a single am plate, and each has a dent depth of 0.
It is possible to reduce the thickness by 0.05 layers or 0.1 layers.

Example 2 Steel having the chemical composition and plate thickness shown in Table 1 was made into a slab by continuous casting, and then hot-rolled to a thickness of 4 mm at a finishing temperature of 870°C.
Cold rolling with a rolling reduction of 80% followed by box annealing at 680°C.
It was finished to a thickness of 0.8 mm by temper rolling with a rolling reduction of 1.0%. Steels A and B are single plates for comparison. Table 1 also shows the tensile property values and dent depths (dent resistance) of these test materials. Steel A has low yield strength, large total elongation, and good workability and surface strain resistance, but its low strength makes it poor in dent resistance, and steel B has good dent resistance, but yield strength is high, the total elongation is large, and the workability and surface strain resistance are insufficient. In contrast to these, the present invention fi
C, D, E, and F have good dent resistance, workability, and surface distortion resistance, and have a good balance of characteristic values not found in conventional steels.

Effects of the Invention Cold-rolled steel sheets are consumed in large quantities for automobiles, electrical products, and building materials, and are also a material for surface-treated steel sheets.The present invention relates to cold-rolled steel sheets used for such purposes, and is described in detail above. As described above, this steel plate has excellent workability, dent resistance, and surface strain resistance that conventional steels lack, and is expected to have extremely large industrial effects.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of the plate thickness composition ratio of the clad steel plate of the present invention. FIG. 2 is a longitudinal sectional view showing an example of an apparatus for implementing the present invention. FIG. 3 is a perspective view showing an outline of the test method in Example 1. FIG. 4 is a graph showing the strength and dent depth of the outer steel plate. 1 Pot φ/Mold, 2. 3. Immersion nozzle, 4. Molten metal pool, 5. Surface layer, 51. Outer layer, 6.
- Inner layer solidification part, 61... Inner layer, 7.6. magnet, 800
．． Slab, 9°0. Punch, 10°...Sample, Representative Patent Attorney Masaru Inoue Ikusuke 2 flashes 1 diagram outside

Claims (2)

[Claims] (1) Components of inner layer in weight%, C: 0.01 to 0.10
%, Mn: 0.05-1.5%, Al: 0.01-0.
10%, P: 0.15% or less, Si: 0.4% or less, S as impurities is 0.02% or less, N is 0.0080% or less, the balance is Fe and unavoidable impurities, and the components of the outer layer are is weight%, C: 0.01-0.15%, Mn: 0
．． 05-2.0%, Al: 0.01-0.10%, P:
0.20% or less, Si: 0.5% or less, S as impurities is 0.02% or less, N is 0.0080% or less, and the balance is Fe and unavoidable impurities. A clad steel plate with good formability and excellent dentability and surface distortion resistance. (2) The total thickness of both outer layers is 10 to 40% of the total thickness, the tensile strength of the outer layer is 35 to 55 kgf/mm^2, the tensile strength of the inner layer is 25 to 45 kgf/mm^2, and the tensile strength of the outer layer is 35 to 55 kgf/mm^2, and The tensile strength is 10kgf/mm^2 higher than the tensile strength of the inner layer.
A clad steel plate with good formability and excellent dent resistance and surface strain resistance as claimed in claim 1, which has higher dent resistance and surface strain resistance.