JPH0639655B2 - Good formability composite steel sheet with excellent burr resistance during press forming and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a composite steel sheet that reduces burrs generated at the end surface of the steel sheet when the steel sheet is pressed, and a method for manufacturing the same.

(Prior Art) Conventionally, a method for producing a composite metal material by continuous casting is known, and is disclosed in, for example, Japanese Patent Laid-Open No. 63-108947. However, the method relates to a continuous casting method for a composite metal material, and does not relate to a method for manufacturing a composite steel sheet having excellent burr resistance during press forming as in the present invention.

Press forming is widely adopted for forming cold-rolled steel sheets into automobile inner plates and outer plates. When performing this press processing,
Attention has been paid to the "burrs" that occur on the edges of steel plates.

That is, when a steel plate is punched and sheared using a tool as shown in FIG. 2, the end surface of the steel plate has a cross-sectional shape as shown in FIG. 3, but at this time, the lower part of the end surface (B portion in FIG. 3). The protrusions formed on the surface are called "burrs".

Generally, when manufacturing a vehicle body of an automobile, first, a press process of several steps including "drawing", "shearing" and "bending" is performed in order to form a steel plate into a predetermined part. The obtained molded product is then assembled into a vehicle body through the steps of "joining" and "painting". Therefore, when a part is coated with a large burr generated during shearing, the burr part does not have a sufficient coating film, and rust is generated from this part, which shortens the life of the automobile.

Therefore, "reducing burrs" has become a major issue in rust prevention measures for automobiles, but in the past, most of them are processing techniques for reducing burrs or methods for removing burrs. Clearance, shear rate, number of punches, material of cutting tool, etc. have been studied by press working technology, but this is not enough measure. That is, there is no effective means for removing burrs, and it is the actual situation that deburring work is performed mechanically and manually, which causes a problem of increasing the number of pressing steps and making it extremely troublesome.

(Problems to be solved by the invention) Therefore, in order to solve the above problems, the inventors of the present invention have repeatedly studied cold-rolled steel sheets with few burrs, and have focused on the relationship between burrs and steel sheet hardness to obtain the optimum hardness. It was found that the composite steel sheet with distribution was good.

That is, an object of the present invention is to provide a good workability composite steel sheet having excellent burr resistance during press forming and a method for producing the same.

(Means for Solving the Problems) In order to achieve the above object, the gist of the present invention is as follows.

(1) The surface layer part is wt%, and contains C 0.01 to 0.15% Mn 0.1 to 2.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less C + Si / 24 + Mn / 4 ≧ 0.3 and the balance Fe And unavoidable impurities, and the internal content is% by weight, C 0.10% or less Si 0.5% or less Mn 0.1 to 1.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less Ti 0.10% or less B 0.001% Below, C + Si / 24 + Mn / 4 <0.3 is included, the balance is Fe and unavoidable impurities, the average hardness of the surface layer portion within 15% of the plate thickness is Hv = 140 to 200, and the internal average hardness is Hv = Composite steel sheet with excellent burr resistance during press forming, which is characterized by being 50 to 130.

(2) One or both of the surface layer and the inside contains Cr of 1.0% or less, and the surface layer contains Nb 0.005 to 0.2% and Ti 0.005.
0.1 to 0.2% of the composite steel sheet having excellent burr resistance during press forming according to the above item 1, characterized by containing one or more of them.

(3) In continuous casting, the surface layer portion is wt% and includes C 0.01 to 0.15% Mn 0.1 to 2.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less C + Si / 24 + Mn / 4 ≧ 0.3 The balance consists of Fe and unavoidable impurities, and the inside is weight%, C 0.10% or less Si 0.5% or less Mn 0.1 to 1.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less Ti 0.10% Below B 0.001% or less A steel slab containing C + Si / 24 + Mn / 4 <0.3 with the balance Fe and inevitable impurities is manufactured, and the steel slab has a finishing temperature of 800 ° C or higher and a coiling temperature of 750 ° C.
Hot rolling is performed below, followed by cold rolling, box annealing or recrystallization annealing with continuous annealing,
Manufacture of a composite steel sheet having excellent burr resistance during press forming, characterized in that the average hardness of the surface layer portion within 15% is Hv = 140 to 200 and the internal average hardness thereof is Hv = 50 to 130. Method.

(4) One or both of the surface layer portion and the inside contains Cr of 1.0% or less, and the surface layer portion contains Nb 0.005 to 0.2% Ti, 0.005 to
0.2% of 1 type (s) or 2 or more types are contained, The manufacturing method of the composite steel plate excellent in the burring resistance at the time of press molding of the said claim | item 3 characterized by the above-mentioned.

(Working) The composite steel sheet of the present invention is a steel sheet characterized in that the surface layer hardening of the steel sheet makes burrs during shearing extremely small and the inside has a soft hardness distribution so that press workability is not impaired. is there.

The effect of surface hardening is that the surface hardening deteriorates the ductility of the surface, and the stress concentration at the initial stage of shearing causes cracks to reduce burrs. However, in the case where the surface layer is not hardened, the surface layer has good ductility, so that the material is elongated by the shearing tension and the burr becomes large.

In the present invention, as shown in FIG. 1, the surface layer average hardness up to 15% of the plate thickness is Hv = 140 to 200, and the internal average hardness is limited to Hv = 50 to 130. The reasons for the limitation will be described below.

The effect of steel sheet properties on burrs during shear punching was investigated using steel sheets with various steel sheet surface characteristics that are commonly used for press working.

As a required characteristic of a steel sheet that does not have burrs, the target is a cold-rolled steel sheet that is normally used for press work with a clearance of 30% on one side and a burr height of 50 μm or less (current material: 130 μm).

In the present invention, the average hardness of the surface layer portion within 15% of the plate thickness is set to Hv = 140 or more in order to harden the surface and set the burr height during shearing to 50 μm or less. On the other hand, the reason why the upper limit of the average hardness of the surface layer portion is set to Hv = 200 is that if the surface is made harder than this, the moldability may be impaired. Also, the internal average hardness was set to Hv = 50 or more because the burr height during shearing is 50 μm when the hardness is less than this.
Because it exceeds. On the other hand, the upper limit of the internal average hardness is Hv
The reason why it is set to 130 is that if it is made harder than this, the moldability may 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 burr on the end surface of the plate after shearing.

The reasons for limiting the composition of the steel for imparting burr resistance to the steel sheet according to the present invention are as follows. All the percentages used in the following description are weight percentages.

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

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

Si is an element that hinders the chemical processability when added too much, and even if added, it should be less than 0.05% or less.

If Mn is too much, the weldability deteriorates, so it is made 2.0% or less. The lower limit is preferably 0.1% or less to prevent S embrittlement.

If P and S are contained in a large amount, press molding is impaired, so the smaller the better, the better the upper limit was made 0.03%.

Al is an element necessary for non-aging, but if it is less than 0.01%, its effect cannot be expected. However, inclusion in a large amount causes the formation of inclusions, so it should be 0.07% or less.

The smaller the content of N, the smaller the amount of addition of the carbide forming element, and therefore the upper limit of N is set to 0.008%.

Cr is an important element in the dual phase steel, but if it exceeds 1.0%, a dual phase steel cannot be obtained, so it is limited to 1.0% or less.

When Nb and Ti are lower than the lower limit, the effect of strengthening is small, and when they are higher than the upper limit, they are saturated. That is, Nb was 0.005 to 0.2% and Ti was 0.005 to 0.2%.

It is limited to C + Si / 24 + Mn / 4> 0.3. If 0.3% or less, Hv = 140 to 200 cannot be obtained.

The inside is Al-killed steel and Ti-killed steel, the constituent elements of which are described below.

If C exceeds 0.10%, the amount of C is set to 0.10% or less in order to obtain a steel sheet that is difficult to non-age even if it is over-aged during continuous annealing. There is a need to.

If Si increases, it hardens and the workability deteriorates, so 0.5
Must be less than or equal to%.

Since Mn deteriorates the r value, it must be 1.0% or less. The lower limit is 0.05% or more to prevent hot brittleness of S.

Al is an element necessary for non-aging, but if it is less than 0.01%, its effect cannot be expected. However, if contained in a large amount, it hardens and the press formability is impaired, so it should be 0.07% or less.

The lower the content of P and S, the softer the content becomes. Therefore, the upper limit of each content is set to 0.03%.

N combines with Al to form AlN and improves the press formability. However, if it exceeds 0.008%, the amount of AlN increases too much and the press formability deteriorates. Therefore, the amount of N is made 0.008% or less.

Ti is an element that improves the press formability, but if it is contained in a large amount, the precipitation strengthening element becomes large and the quality of the material deteriorates, so it is made 0.10% or less.

B is an essential element for improving the secondary workability.
However, if contained in a large amount, it hardens and the press moldability is impaired, so the content was made 0.001% or less.

Cr improves stretch flangeability, but if too much is added, ductility deteriorates. The upper limit is 1.0% or less.

It is limited to C + Si / 24 + Mn / 4 <0.3. If it exceeds 0.3, Hv = 50 to 130 cannot be obtained.

The steel having the above-described composition is produced by the continuous casting method and sent to the hot rolling process. In the present invention, the finishing temperature of hot rolling is 800 ° C or higher (preferably 870 to 910 ° C) and the coiling temperature is 750. C. or lower (preferably 550 to 750.degree. C.).

Cold rolling is performed after descaling, and the higher the rolling reduction is, the better the deep drawability is. 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 the secondary workability. The annealing temperature is required to be higher than the recrystallization temperature in order to secure the deep drawability. Any cooling method (gas jet method, steam method, roll cooling method, water quenching method, etc.) may be used for cooling after annealing. The overaging treatment temperature is 200 to 500 ° C. The annealed steel sheet is subjected to temper rolling of 5% or less (preferably 0.5 to 1.0%) if necessary and provided as a product.

(Example) The components shown in Table 1 are melted by continuous casting, the slab heating temperature is 1150 ° C or higher, the finishing temperature is 900 to 910 ° C, and the winding temperature is 550 to 7
It was hot rolled at 00 ° C. Box pickling after pickling, cold rolling:
680-700 ℃ × 16 hours, continuous annealing: Soaking 800 ℃ × 1 minute, over-aging treatment: 300 ℃ × 5 minutes, and skin pass
It took 0.8-1.0%.

Table 2 shows the results of the burr height and the hole expansion ratio in the punching of the obtained steel sheet.

The products of the present invention (test steel Nos. 1 to 5) all show good results.

Specimen No. 6 is a comparative example, and although the burr height is as small as 20 μm, it hardens and impairs formability.

The sample steel No. 7 is a comparative example, and the burr height is 120 μm.

(Effect of the invention) According to the present invention, the average hardness of the surface layer portion within 15% of the plate thickness is Hv = 140 to 200, and the internal average hardness thereof is Hv = 50 to 13
By setting it to 0, it is possible to provide a composite steel sheet having extremely excellent burr resistance during press working. ADVANTAGE OF THE INVENTION According to this invention, the rust prevention of the end surfaces of the inner plate and outer plate for an automobile is improved, and the life of the automobile can be significantly extended.

Also, according to the present invention, a composite plate having an optimum hardness distribution and extremely excellent in burr resistance at the time of press molding can be obtained, and a uniform wide material product can be manufactured at low cost.

Further, according to the present invention, it can be used even if a steel plate having an optimum hardness distribution is formed and then plated.

[Brief description of drawings]

Fig. 1 is a schematic diagram showing the distribution of hardness differences within the plate thickness, Fig. 2 is an explanatory diagram showing the method of shearing (punching), and Fig. 3 is a schematic diagram showing the burr form of the plate edge after shearing of conventional steel. FIG. 4 and FIG. 4 are schematic diagrams showing the burr form of the plate edge after shearing of the steel of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nobuhiko Matsuzu 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Inside the Kimitsu Works (72) Inventor Ryuji Takahashi 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Kimitsu Works (72) Inventor Yoshio Hashimoto 1-1-1 Edami, Hachimanto-ku, Kitakyushu, Fukuoka Prefecture Inside the 3rd Technical Research Institute, Nippon Steel Corp. (72) Tomohisa Katayama 5-10 Fuchinobe, Sagamihara City, Kanagawa Prefecture ―1 Nippon Steel Corporation Second Technical Research Center

Claims (4)

[Claims] 1. The surface layer portion is wt%, and contains C 0.01 to 0.15% Mn 0.10 to 2.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less C + Si / 24 + Mn / 4 ≧ 0.3, and the balance. Consists of Fe and unavoidable impurities, and the inside is% by weight, C 0.10% or less Si 0.5% or less Mn 0.1 to 1.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less Ti 0.10% or less B 0.001% or less Containing C + Si / 24 + Mn / 4 <0.3, balance Fe and unavoidable impurities, the average hardness of the surface layer portion within 15% of the plate thickness is Hv = 140 to 200, and its internal average hardness is A composite steel sheet having excellent burr resistance during press forming, characterized by Hv = 50 to 130. 2. Cr on one or both of the surface layer and the inside
1.0% or less, Nb 0.005 to 0.2%, T in the surface layer
The composite steel sheet having excellent burr resistance during press forming according to claim 1, characterized in that it contains one or more of 0.005 to 0.2% of i. 3. In the continuous casting, the surface layer portion is% by weight, C 0.01 to 0.15% Mn 0.1 to 2.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less C + Si / 24 + Mn / 4 ≧ 0.3 The balance is Fe and unavoidable impurities, and the inside is weight%, C 0.10% or less Si 0.5% or less Mn 0.1 to 1.0% P 0.03% or less S 0.03% or less Al 0.01 to 0.07% N 0.008% or less Ti 0.10 % Or less B 0.001% or less A steel slab containing C + Si / 24 + Mn / 4 <0.3 with the balance Fe and unavoidable impurities is manufactured. The steel slab has a finishing temperature of 800 ° C or higher and a coiling temperature of 750 ° C.
Hot rolling is performed below, followed by cold rolling, box annealing or recrystallization annealing with continuous annealing,
Manufacture of a composite steel sheet having excellent burr resistance during press forming, characterized in that the average hardness of the surface layer portion within 15% is Hv = 140 to 200 and the internal average hardness thereof is Hv = 50 to 130. Method. 4. A Cr layer on one or both of the surface layer and the inside
Contains 1.0% or less, and Nb 0.005 to 0.2% in the surface layer,
4. The method for producing a composite steel sheet having excellent burr resistance during press forming according to claim 3, wherein one or more of Ti 0.005 to 0.2% is contained.