JPH0356644A - Clad steel sheet excellent in burr resistance at the time of press forming and its production - Google Patents

Abstract

PURPOSE:To produce a clad steel sheet excellent in burr resistance at the time of press forming by continuously casting a steel slab consisting of a surface layer part and an inner part having respectively specified compositions, subjecting the cast slab to specific hot rolling and then to cold rolling and recrystallization annealing, and regulating the hardness in the surface layer part and the hardness in the inner part to specific values, respectively. CONSTITUTION:A steel slab which consists of a surface layer part having a composition consisting of, by weight, 0.01-0.15% C, <=0.30% Si, <=2.5% Mn, <=0.15% P, <=0.03% S, 0.01-0.07% Al, <=0.10% Ti, <=0.008% N, and the balance Fe with inevitable impurities and further containing, if necessary, <=0.1% Nb or rare earth metal and an inner part having a composition consisting of <=0.08% C, <=0.03% Si, <=0.40% Mn, <=0.03% P, <=0.03% S, 0.01-0.07% Al, <=0.008% N, <=0.10% Ti, <=0.001% B, and the balance Fe with inevitable impurities is prepared by means of continuous casting. The above cast slab is hot-rolled at >=800 deg.C finishing temp. and <=750 deg.C coiling temp. Subsequently, the resulting steel plate is cold-rolled and subjected to recrystallization annealing. By this method, the average hardness in the surface layer part within 15% of the plate thickness and the average hardness of the inner part are regulated to Hv140-200 and Hv50-100, respectively, by which the clad steel sheet excellent in burr resistance can be obtained easily and inexpensively.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a composite steel plate and a method for manufacturing the same, which reduce the amount of flash generated on the end face of a steel plate when the steel plate is press-formed.

(Prior Art) Conventionally, a method of manufacturing a composite metal material by continuous casting is known, and is disclosed, for example, in Japanese Patent Application Laid-open No. 108947/1983. However, this method is related to a continuous casting method for composite metal materials, and is not related to a method for manufacturing a composite steel sheet with excellent burr resistance during press forming as in the present invention.

Pressing is widely used to form cold-rolled steel sheets into automobile interior and exterior panels. When performing this press processing,
``R burrs that occur on the edge of steel plates'' are attracting attention. That is, when a steel plate is punched and sheared using a tool as shown in Fig. 2, the end face of the steel plate has a cross-sectional shape as shown in Fig. 3, but at this time, the lower part of the end face (section B in Fig. 3) The protrusions that appear on the surface are called ``burrs.'' Generally, when manufacturing an automobile body, first, in order to shape a steel plate into a predetermined part, "restriction 1, r shear 1. Several steps of press working, including r-bending 1, are performed. The obtained molded product is then
It is assembled into the vehicle body through the following processes: ``bonding'' and ``painting''. Therefore, if a part is painted with large flakes generated during shearing left on the part, the paint film will not be sufficiently attached to the flakes, causing rust to form in these areas and shortening the life of the car. Therefore, ``reducing burrs has become a major issue in rust prevention measures for automobiles.''

Most of the conventional research has focused on processing techniques to reduce the size of the particles or methods for removing the particles. In press processing technology, clearance and shear rate. Number of punchings. Although the material of the cutlery has been studied, it is not sufficient.

There is no very effective method for removing pars, and the process requires mechanical or manual deburring, which increases the number of pressing steps and is very time-consuming.

(Issue 8 to be solved by the invention) Therefore, the present inventors have conducted repeated research on cold-rolled steel sheets with low occurrence of paris, and focused on the relationship between occurrence of paris and steel sheet hardness, and have developed a composite steel plate with an optimal hardness distribution. It was found to be good.

(Means for solving problems) The gist of the present invention is as follows.

(11 The surface layer is in weight%, C 0.01 -0.15χ St 0.30X or less Mn 2.5χ or less P 0.15χ or less S 0.03χ or less Al 0.01 -0.07χ Ti 0.10X or less Contains N 0.008χ or less, the remainder consists of Fe and unavoidable impurities, the inside is in weight%, C 0.08χ or less Si 0.03χ or less Mn 0.40χ or less P 0.03χ or less S 0.03χ or less Al 0.01~0.07χ N 0.008X or less Ti 0.10χ or less B 0.OOIX or less, the balance is Fe and unavoidable impurities, and the average hardness of the surface layer within 15% of the plate thickness is Hv= 140-2
00, and its internal average hardness is Hv = 50 ~
100. A composite steel plate with excellent burr resistance during press forming.

(2) The composite steel sheet with excellent burr resistance during press forming according to item 1 above, wherein the surface layer further contains 0.1% or less of Nb and one or more rare earth metals.

(3) In continuous casting, the surface layer part has a C 0.01 to 0.01% by weight. 15χ St 0.30X or less Mn 2.5X or less P 0.15χ or less S 0.03X or less AI 0 01 ~ 0.07χ Ti 0.10χ or less N 0.008χ or less, the remainder consists of Fe and unavoidable impurities , inside is weight %, c o. osx or less St 0.03χ or less Mn 0.40% or less P 0.03χ or less S 0.03χ or less Al 0.01 to 0.07χ N 0.008X or less Ti 0.10χ or less B 0.001χ or less, the remainder A steel billet made of Fe and unavoidable impurities is produced, and the steel billet is heated to a finishing temperature of soo'c or higher and a standard temperature of 75
By performing hot rolling at 0°C or lower, followed by cold rolling, and recrystallization annealing by box annealing or continuous annealing, the average hardness of the surface layer within 15% of the plate thickness is reduced to Hv = 14.
0 to 200, and the internal average hardness is Hv=50 to
100. A method for manufacturing a composite steel plate having excellent burr resistance during press forming.

(4) The method for producing a composite steel sheet with excellent burr resistance during press forming according to item 3 above, wherein the surface layer further contains 0.1% or less of Nb and one or more rare earth metals. .

(Function) The composite steel plate of the present invention is a steel plate characterized in that the surface hardening of the steel plate makes the cracks during shearing extremely small, and the interior has a soft hardness distribution so that press workability is not impaired.

The effect of surface hardening is that the ductility of the surface layer deteriorates due to surface hardening, cracks occur due to stress concentration at the initial stage of shearing, and the paris becomes smaller. However, in the case of materials without surface hardening, the surface layer has good ductility, so the material is stretched by shearing tension and the crack becomes large.

In the present invention, as shown in FIG. 1, the average hardness of the surface layer within 15% of the plate thickness is set to Hv=140-200, and the average internal hardness is limited to Hv=50-100. The reason for this limitation will be explained below. We investigated the influence of steel sheet properties on paris during shear punching using composite steel sheets that are normally subjected to press working, with various surface characteristics.

The required properties for a steel plate that does not produce any cracks are a clearance of 30% on one side and a clearance height of 50 μm or less (current material is about 130 μIIl) for cold-rolled steel sheets that are normally subjected to press working. In this invention, the average hardness of the surface layer within 15% of the plate thickness is set to Hv-140 or higher in order to harden the surface and reduce the par height during shearing to 50 μm or less. On the other hand, the upper limit of the average hardness of the surface layer is set to Hv-200 because if the surface is made harder than this, there is a risk that the appearance will be impaired. The reason why we set the internal average hardness to Hv=50 or more is that if the hardness is less than this, the par height during shearing will be 5.
This is because it exceeds 0Ilffi. On the other hand, the reason why the upper limit of the internal average hardness is set to llv=100 is that if it becomes harder than this, there is a risk that moldability will be impaired.

As described above, according to the present invention, it is possible to provide a steel plate with extremely small cracks at the end face of the plate after shearing, as shown in FIG.

According to the present invention, the reasons for restricting the precepts of steel for imparting burr resistance to a steel plate are as follows. Note that all percentages used in the following explanation are htχ.

The surface layer is made of high-strength steel, and its constituent elements will be described below.

C is an important element for surface hardening, but if it is less than 0.01%, surface hardening is difficult. However, if it exceeds 0.15%, spot weldability is impaired, so the upper limit is set at 0.15%.

Since Si is an element that inhibits chemical processability if added too much, it is necessary to limit the amount to 0.3% or less.

Mn is an important element for surface hardening, but since it deteriorates spot weldability, it is limited to 2.5% or less.

P is also an important element for surface hardening, but if it exceeds 0.15%, the ductility of the steel plate will deteriorate significantly, so it is necessary to keep it below 0.15%.

If S is included in a large amount, the breathability will be impaired, so the smaller the amount, the better, so the upper limit was set at 0.03%.

M is an element necessary for non-aging, but 0. The effect cannot be expected if it is less than Ol%. However, if it is included in a large amount, it may cause inclusions, so it should be kept at 0.07% or less.

Ti forms fine carbonitrides and is an important element for surface hardening. However, since adding a large amount increases the cost, the upper limit is set at 0. It was set at 10%. Also, as the amount of Ti decreases, the effect of hardening the surface layer decreases, so the lower limit is set to o. o
It is preferable to set it as os%.

The lower the amount of N, the less carbide-forming elements need to be added, so the upper limit was set at 0.0Q8%.

Nb is also effective in hardening the surface, but excessive addition impairs pressability, so it was limited to 0.1% or less.

Rare earth metals are effective in improving press forming properties, but excessive addition increases costs, so the upper limit was set at 0.1%.

The interior is made of M-killed steel and Ti-killed steel, the constituent elements of which are described below.

When C exceeds 0.08%, it is difficult to make the steel non-aging even if an overaging treatment is performed during continuous annealing. In order to obtain a steel plate that is non-aging and has excellent deep drawability, it is necessary to keep Cl at 0.08% or less.

If Si increases, it will harden and deteriorate workability, so 0.
It is necessary to keep it below 0.3%.

Since Mn deteriorates the r value, it needs to be kept at 0.40% or less.

Although M is a necessary element for non-aging, the effect cannot be expected if it is less than 0.01%. However, if it is contained in a large amount, it becomes hard and press formability is impaired, so it should be kept at 0.07% or less.

As for P and S, the lower the content, the softer the material becomes, so the upper limit of each was set at 0.03%.

N combines with M to form AfN and improves press formability, but if it exceeds 0.008%, the amount of AjN increases too much and press formability deteriorates, so Nl is set to 0.008% or less. .

Ti is an element that improves press forming properties, but if it is included in a large amount, precipitation strengthening factors will become large and the quality of the material will deteriorate, so it should be set at 0. 10% or less.

B is an essential element for improving secondary workability.

However, if it is contained in a large amount, it will become hard and the press shape will be impaired, so it is set at 0.001% or less.

Copper having the above composition is manufactured by continuous Vt casting and sent to a hot rolling process. In the present invention, the finishing temperature of hot rolling is 800°C or higher (preferably 870 to 910°C).
T? Winding temperature 750'C or less (preferably 550-75
0"C). After descaling, cold rolling is performed, and the higher the rolling reduction, the better for improving deep drawability, and is preferably <75% or more. Next, regarding the annealing conditions, the annealing method is continuous. Although annealing is performed using the annealing method or box annealing method, continuous annealing is more preferable for improving secondary workability.The annealing temperature must be higher than the recrystallization temperature to ensure deep drawability. Cooling after annealing may be done by any method (gas jet method, air water method, roll cooling method, water quenching method, etc.).

Moreover, the overaging treatment temperature is 200 to 500°C. The annealed steel plate may have a content of 5% or less (preferably 0.5~
1.0%) and is then subjected to temper rolling and provided as a product.

(Example) The components shown in Table 1 were melted by continuous casting, and hot rolled at a slab heating temperature of 1150°C or higher, a finishing temperature of 900 to 910'C, and a winding temperature of 550 to 700°C. After pickling and cold rolling, box annealing at 680-700'C for 16 hours, continuous annealing: Soaking at 800°C for 1 minute, over-aging treatment at 300°C
Each was applied for 5 minutes and a skin pass was applied at a rate of 0.8 to 1.0%.

Table 2 shows the results of the crisp height of the punched steel sheet obtained.

All of the products of the present invention (sample steel Nos. 1 to 5) show good results.

Test! l! INα6 is a comparative example and has a small par height of 20 mm, but it becomes hard and impairs formability.

Sample mNll7 is a comparative example, and the Paris height is 120 μm.
It is.

(Effect of the invention) According to the present invention, the average hardness of the surface layer up to 15% of the plate thickness is set to }Iv = 140 to 200, and the internal average hardness is set to Hv = 50 to 100, thereby improving burr resistance during press working. It is possible to provide composite steel sheets with extremely excellent properties.

According to the present invention, the end face protection of the inner and outer panels of an automobile is improved, and the life of the automobile can be greatly extended.

Further, according to the present invention, a composite steel plate having an optimum hardness distribution and extremely excellent burr resistance during press forming can be easily obtained, and a homogeneous wide material product can be manufactured at low cost.

Further, it is also possible to use a steel plate having an optimum hardness distribution according to the present invention and then subjecting it to plating treatment.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the hardness difference distribution within the plate thickness, Figure 2 is an explanatory diagram showing the shearing (punching) method, and Figure 3 is a schematic diagram showing the shape of the edge of the plate after shearing of conventional steel. FIG. 4 is a schematic view showing the shape of the edge of the sheet after shearing of the steel of the present invention -260-. Little brother l Figure 2 Figure dice

Claims (4)

[Claims] (1) The surface layer is expressed in weight percent: C0.01~0.15% Si0.30% or less Mn2.5% or less P0.15% or less S0.03% or less Al0.01~0.07% Ti0.10% Contains N0.008% or less, the remainder consists of Fe and unavoidable impurities, the inside is expressed in weight%, C0.08% or lessSi0.03% or lessMn0.40% or lessP0.03% or lessS0.03% or less Al: 0.01 to 0.07% N: 0.008% or less Ti: 0.10% or less: B: 0.001% or less; the remainder consists of Fe and unavoidable impurities; the average hardness of the surface layer within 15% of the plate thickness is Hv =140~2
00, and its internal average hardness is Hv=50 to 10
A composite steel plate with excellent burr resistance during press forming, characterized by having 0. (2) The composite steel sheet with excellent burr resistance during press forming according to claim 1, wherein the surface layer further contains 0.1% or less of Nb and one or more rare earth metals. (3) In continuous casting, the surface layer is expressed in weight percent: C0.01-0.15% Si0.30% or lessMn2.5% or lessP0.15% or lessS0.03% or lessAl0.01-0.07%Ti0 .10% or less N0.008% or less, the balance consists of Fe and unavoidable impurities, the inside is in weight%, C0.08% or lessSi0.03% or lessMn0.40% or lessP0.03% or lessS0. 03% or less N0.01~0.07% N0.008% or less Ti0.10% or less B0.001% or less, and the balance is Fe and unavoidable impurities. ℃ or higher, winding temperature 750
By performing hot rolling at a temperature below ℃, followed by cold rolling, and recrystallization annealing by box annealing or continuous annealing, the average hardness of the surface layer within 15% of the plate thickness is reduced to Hv = 140 to 2.
00, and its internal average hardness is Hv=50-100
A method for producing a composite steel sheet with excellent burr resistance during press forming, characterized by: (4) The method for manufacturing a composite steel sheet with excellent burr resistance during press forming according to claim 3, wherein the surface layer further contains 0.1% or less of Nb and one or more rare earth metals. .