JPH0432538A - High r-value electrogalvanized clad steel sheet excellent in burring resistance and electrogalvanizing adhesion and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a steel sheet excellent in burring resistance, electrogalvanizing adhesion, etc., at the time of press forming by rolling a slab in which each compsn. of the surface layer part and inside is regulated, thereafter subjecting it to recrystallization annealing, then executing electrogalvanizing and specifying each hardness of the surface layer part and inside in the sheet. CONSTITUTION:A slab in which the surface layer part is constituted of, by weight, <0.01% C, 0.05 to 2.5% Mn, 0.05 to 0.10% P, <=0.03% S, 0.01 to 0.07% Al, 0.03 to 0.05% Ti, <=0.008% N and the balance iron with inevitable impurities as well as the inside is constituted of <=0.08% C, <=0.03% Si, <=0.40% Mn, <=0.03% P, <=0.03% S, 0.01 to 0.07% Al, <=0.008% N, <=0.10% Ti, <=0.001% B and the balance iron with inevitable impurities is manufactured by continuous casting. This slab is hot-rolled at >=800 deg.C finishing temp. and <=750 deg.C coiling temp., is thereafter cold-rolled and is subjected to recrystallization annealing by box annealing or continuous annealing. Successively, this slab is subjected to electrogalvanizing in an electrogalvanizing line to regulate the average hardness of the surface layer part to <=15% of the sheet thickness to 140 to 200Hv and that at the inside to 50 to 100Hv, by which the above steel sheet can be obtd.

Description

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

(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. 1983-10111947. 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 plate with excellent stiffness 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,
The occurrence of cracks on the edges of steel plates is 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 paris.

Generally, when manufacturing an automobile body, a steel plate is first subjected to several press processes including drawing, shearing, and bending in order to form it into a predetermined part.

The obtained molded product is then assembled into a vehicle body through bonding and painting steps.

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 the size of the cracks 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, there is insufficient consideration of clearance, shearing speed, number of punches, material of the cutter, etc.

Not much effective te on how to remove Paris.

There were no steps, and the deburring work was done manually and mechanically, which increased the number of pressing steps and was extremely time-consuming.

(Problems to be Solved by the Invention) The present inventors have repeatedly conducted research on cold-rolled steel sheets with little occurrence of firmness, and have found that composite steel plates with an optimal hardness distribution in terms of the relationship between occurrence of firmness and steel plate hardness are good. I discovered something.

(Means for Solving the Problems) The gist of the present invention is (1) the surface layer is % by weight;
C0,0] less than 9o, Fvl n 0 , 05~2,
596, PO005 ~ 0.10%, 30.03% or less,
A, Q 0.01~007%, Tj0.03~0.05
96, contains 0.008% or less of N, the remainder consists of Fe and unavoidable impurities, the internal content is % by weight, C0.0g9
6 or less, Si 0.03% or less, Mn 040% or less, Po
, 039° or less, S 0.03% or less, A[0,01-
0,0796, N 0.008% or less, Ti 0.10
% or less, B 0.001% or less, the balance consists of Fc and unavoidable impurities, the average hardness of the surface layer within 15% of the plate thickness is Hv = 140 to 200, and the internal average hardness is Hv = 50 to 100, and (2) a high r-value electrogalvanized composite steel sheet with excellent stiffness resistance during press forming, and (2) continuous casting, the surface layer has a C
Less than 0.01%, M n 0 , 05-2, 596,
PO105~0.10%, S0.03% or less, Ag0.
01-0, 0796, T10.03-0.05%, N
Contains 0.008% or less, the balance consists of Fe and unavoidable impurities, the internal content is 0.08% or less by weight, S
i0.03% or less, Mn0140% or less, Po, 039
ci or less, S 0.03% or less, 8Ω 0.01 to 0.0
79ci, N 0.008% or less, Ti 0.10%
Hereinafter, a steel billet containing 0.001% or less of B, the balance consisting of Fe and unavoidable impurities is produced, and the steel billet is finished at a finishing temperature of 8.
Hot rolling is performed at a rolling temperature of 00°C or higher and a winding temperature of 750°C or lower, followed by cold rolling, recrystallization annealing by box annealing or continuous annealing, and then electrogalvanizing on an electrogalvanizing line to reduce the plate thickness. Excellent burr resistance and plating adhesion during press molding, characterized by having an average hardness of the surface layer within 15oo of Hv = 140 to 200, and an average internal hardness of Hv = 50 to 100. This is a method for producing a high r-value electrogalvanized composite steel sheet.

(Function) What about the present invention? U=Steel plate is a steel plate characterized by surface hardening of the steel plate to extremely reduce cracking during shearing, and a soft internal hardness distribution that does not impair press workability.

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 firmness decreases. However, materials without surface hardening have good surface ductility, so the material is stretched by the shearing tension, resulting in a larger break.

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 to 200, and the internal average hardness is limited to Hv-50 to 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 flakes are a clearance of 300 degrees on one side and a stiffness height of 50 μm or less (current material is approximately 130 μm) for cold-rolled steel sheets that are normally subjected to press working.

In this invention, the average hardness of the surface layer up to 1598 mm of the plate thickness is set to Hv=140 or more in order to harden the surface and make the firmness height at 1-shearing 50 μm or less.

On the other hand, the reason why the upper limit of the uniform hardness of the surface layer part is set to Hv=200 is that if the surface is made harder than this, there is a risk that the moldability will be impaired.

The reason why the internal hardness was set to Hv=50 or more is because if the hardness was less than this, the height of the burr during shearing would exceed 50 μm.

On the other hand, the upper limit of the internal average hardness was set to Hv=100 because
This is because if the material is made harder than this, there is a risk that moldability will be impaired.

As described above, according to the present invention, as shown in FIG. 4, it is possible to provide a steel plate with extremely small cracks at the end face of the plate after shearing.

According to the present invention, the reasons for limiting the components of the steel for imparting stiffness resistance to the steel plate are as follows. Note that all percentages used in the following explanation are Wt9o.

The surface layer is made of high-strength steel, and 3C, which is a constituent element of the steel, is an element that causes aging phenomena in cold-rolled steel sheets, so it is necessary to suppress its content as much as possible. 001%
The content was regulated to less than

Since St is an element that inhibits chemical processability if added in excess, even if it is unavoidably present, it must be kept at less than 0.05%.

Since Mn is a heavy element for surface hardening and deteriorates spot weldability, the upper limit is limited to 2.5% or less. The lower limit is S
The content is preferably 0.05% or more to prevent embrittlement.

Since P is also an important element for surface hardening, 0.059 ci or more is required. If it is added too much, the ductility of the steel plate will be significantly deteriorated, so it is necessary to keep it below Io''o.

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

Although Ag is a necessary element for non-aging, the effect cannot be expected if it is less than 0.01%. However, if it is included in many layers, it will cause the formation of inclusions, so it should be 0,074' (1 or less).

Tj forms fine carbonitrides and is an important element for surface hardening. However, since adding a large amount not only increases cost but also makes it unfavorable for galvanizing properties, the upper limit was set at 0.05%. Further, as the amount of Ti decreases, the effect of hardening the surface layer decreases, so it is preferable that the lower limit is 0.03%.

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

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

If 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 reduce the amount of C to 0.08% or less.

If Si increases, it will harden and deteriorate workability, so 0.
Is it necessary to lower it to 0.3% or less?

Since Mn deteriorates the r value, it is necessary to set it to 0.4006 or less.

Is Aρ a necessary element for non-aging? 0,01
If it is less than 9ci, the effect cannot be expected. However, if it is contained in a large amount, it will become hard and the press formability will be impaired, so 0.
It should be 0.7% or less.

As for PS, the lower the content, the softer it becomes, so the upper limit of each was set to 0.030o.

Either N combines with A and Q to form AΩN and improves press formability, or if it exceeds 0.00896, AN Nm increases too much and press formability deteriorates, so the amount of N is reduced to 0.
008% or less.

Ti is an element that improves press formability, and if it is included in a large amount, the precipitation strengthening factor becomes large and the quality of the material deteriorates, so the content is set to 0.10% or less.

B is an essential element for improving secondary workability.

However, if it is contained in a large amount, it becomes hard and press formability is impaired, so the content was set at 0.001% or less.

Steel having the above composition is manufactured by a continuous casting method and sent to a hot rolling process, or in the present invention, the finishing temperature of hot rolling is 800°C or higher (preferably 870 to 910°C).
The winding temperature is 750℃ or less (preferably 550 to 750℃)
).

After descaling, cold rolling is performed, and the higher the rolling reduction, the better for improving deep drawability, and preferably 759 ci or more.

Next, regarding the annealing conditions, the annealing method is a continuous annealing method or a box annealing method, but the continuous annealing method is more preferable for improving secondary workability. The annealing temperature must be recrystallized at 73 degrees or higher to ensure deep drawability.

Cooling after annealing may be carried out by any method (gas shed method, air water method, roll cooling method, water quenching method, etc.).

Further, the overaging treatment temperature is 200 to 500°C.

The annealed steel plate may have a content of 5% or less (preferably 0.0%) if necessary.
5 to 1.09 ci) is subjected to temper rolling.

Next, the production of electrogalvanizing is not particularly limited; for example, the composition of the zinc plating bath is 380 g/I of zinc sulfate, 72 g/II of sodium sulfate, and 61 g/g of magnesium sulfate, and the pH of the bath is 3.0 to 40. Warm 55~
An electrogalvanized steel sheet can be obtained by manufacturing under the conditions of 65° C. and cathode current density of 25-40 A/dm'.

(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 %0 to 910°C, and a winding temperature of 550 to 700°C.

After pickling and cold rolling, box annealing at 680-700°C
Continuous annealing for 16 hours, soaking at 800°C for 5 minutes, over-aging treatment at 300°C for 5 minutes, and a skin bath of 0.
After applying 8~10°0, plate it on the electroplating line 5
．． Q20g/rT'It was applied to produce a product.

? Table 2 shows the results of the cutting height, plating characteristics, and r value of the stamped steel plate.

All of the products of the present invention (sample steels Nos. 011 to 5) show good results.

Test steel No. 6 is a comparative example, and the stiffness height is 20 μm.
Although it is small, it becomes hard and impairs moldability.

Test w4 No. 7 is a comparative example, and the firmness height is 120
It is μm.

(Effect of the invention) According to the present invention, the average hardness of the surface layer within 15% of the plate thickness is set to Hv-140 to 200, and the internal average hardness is set to Hv-140 to 200.
-50 to 100, it is possible to provide a composite steel plate with extremely excellent burr resistance during press working.

According to the present invention, the rust prevention of the end surfaces of the inner and outer panels of automobiles can be 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.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the hardness difference or 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 plate after shearing of the steel of the present invention.

Claims (1)

[Claims] 1. The surface layer portion is in weight percent: C less than 0.01% Mn 0.05-2.5% P 0.05-0.10% S 0.03% or less Al 0.01-0.07% Ti0 .03 to 0.05% N0.008% or less The remainder consists of Fe and unavoidable impurities, and the inside is 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, and the average hardness of the surface layer within 15% of the plate thickness is Hv = 140 to 200, and its internal average hardness is Hv = 50 to 100, and has excellent burr resistance during press molding.
Value electrogalvanized composite steel sheet. 2. In continuous casting, the surface layer is in weight percent: C less than 0.01% Mn 0.05-2.5% P 0.05-0.10% S 0.03% or less Al 0.01-0.07% Ti 0.03- 0.05% 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 A steel billet consisting of Al 0.01 to 0.07% N 0.008% or less Ti 0.10% or less B 0.001% or less the remainder Fe and unavoidable impurities is produced, and the steel billet is heated at a finishing temperature of 800°C or higher and a winding temperature. Hot rolling is performed at 750°C or less, followed by cold rolling, recrystallization annealing is performed by box annealing or continuous annealing, and then electrogalvanizing is performed on an electrogalvanizing line to remove the surface layer up to 15% of the plate thickness. High r-value electrogalvanizing with excellent burr resistance and plating adhesion during press molding, characterized in that the average hardness of the part is Hv = 140 to 200, and the internal average hardness is Hv = 50 to 100. Method for manufacturing composite steel plates.