JPH08127839A - Multiply-layered steel sheet excellent in corrosion resistance and workability and production thereof - Google Patents

Abstract

PURPOSE: To make the corrosion resistance and workability of a steel sheet compatible by specifying the compsn. of a multiply-layered steel sheet and increasing the contents of P and Cu in a specified thickness from the surface and back faces. CONSTITUTION: In a multiply-layered steel sheet in which the components of the surface layer and internal layer are different, the components in the internal layer are constituted of, by weight, 0.01 to 0.20% C, 0.002 to 1.00% Si, 0.02 to 3.0% Mn, 0.002 to 0.030% P, 0.002 to 0.050% S, 0.002 to 0.100% Al, 0.0002 to 0.0100% N, and the balance Fe with inevitable impurities. In this multiply- layered steel sheet, in the case of the steel sheet is defined as (t), the components in the surface layer at the part of 0.015 to 0.15t per side from the surface and rear faces are regulated to the same as those in the internal layer as for C, Si, Mn, S, Al and N, which are incorporated with 0.030 to 0.150% P and 0.15 to 2.0% Cu, and the balance Fe with inevitable impurities. This multiply-layered steel sheet can be obtd. by pouring the internal layer components into a continuous casting mold, applying a static magnetic field at the upper part and adding Cu and P to the steel at the upper part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet used for automobile bodies, home electric appliances, building materials and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Many attempts have been made to improve the corrosion resistance of steel sheets. For example, as shown in Japanese Patent Publication No. 57-14748, there is known a steel obtained by adding Cu and P to a low carbon Al killed steel. ing. However, since such a steel contains a large amount of Cu and P, it has a drawback that the workability is deteriorated although the corrosion resistance is improved.

[0003]

DISCLOSURE OF THE INVENTION The present invention is a multi-layer steel sheet and a method for producing the same, which has been made to solve the drawback that the workability deteriorates although the above steel has improved corrosion resistance.

[0004]

Means for Solving the Problems The gist of the present invention is, in a multi-layer steel sheet having different components in the surface layer and the inner layer, the weight ratio of C: 0.01 to 0.20%, Si: 0.002 to 1.
00%, Mn: 0.02 to 3.0%, P: 0.002 to 0.030%,
S: 0.002-0.050%, Al: 0.002-0.100%, N:
0.0002 to 0.0100%, balance: Fe and unavoidable impurities. If the plate thickness is t, 0.015 t to 0.15 t from the front and back surfaces.
The surface layer component of the part is the same as the inner layer component for C, Si, Mn, S, Al and N, and P: 0.030-0.
150%, Cu: 0.15 to 2.0%, balance: Fe and unavoidable impurities, or Ni: 0.08 to 2.0 as a surface layer component
% Is a multi-layer steel sheet with excellent corrosion resistance and workability.

Further, in the present invention, molten steel having the above-mentioned inner layer component is vertically injected into a mold for continuous casting together with gas,
A static magnetic field is applied to the entire width of the mold in the width direction above the molten steel injection position to decelerate the ascending flow of the steel, and Cu and P or Cu and P and Ni are applied to the steel above the magnetic field application position. Was added so that the molten steel in the upper part becomes the above surface layer composition by stirring the injected gas, and by drawing the mold, a slab having the above component composition was formed, and then hot rolling was performed based on the ordinary method of steel plate production. , Descaling, cold rolling, annealing, the inner layer has the above-mentioned inner layer components, and the plate thickness is t,
A method for producing a multi-layer steel sheet having excellent corrosion resistance and workability, which comprises forming a cold rolled steel sheet containing 0.015 to 0.15 tons of surface layer components from the front and back surfaces.

As described above, the reason why Cu and P are contained only in the surface layer is that Cu and P are elements necessary for improving the corrosion resistance, but Cu and P are elements that increase the strength. When it is contained in a large amount, the strength becomes high and the workability also deteriorates. For this reason, it is not desirable to include it in applications requiring workability.

Therefore, in the present invention, the places where Cu and P are present are limited to only the surface layer portion required for corrosion resistance. Since the inner layer that occupies more than half of the plate thickness does not contain much Cu and P, it is possible to secure the necessary workability as a component,
Further, since the corrosion resistance can be secured in the surface layer portion, it is possible to obtain a steel sheet having both workability and corrosion resistance.

The reasons for limiting the components of the inner layer will be described. C
Is less than 0.01%, the solid solution C in steel increases and the workability deteriorates, so 0.01% is made the lower limit. If it exceeds 0.20%, the amount of carbides increases and the workability deteriorates.
Is the upper limit. If Si is reduced to less than 0.002%, the manufacturing cost will increase dramatically and the economic efficiency will be impaired.
The lower limit is 002%, and if it exceeds 1.0%, the workability deteriorates, so 1.0% is the upper limit.

If Mn is reduced to less than 0.02%, the manufacturing cost will be dramatically increased and the economic efficiency will be impaired.
Is the lower limit, and if it exceeds 3.0%, the workability deteriorates, so 1.
The upper limit is 0%. If P is reduced to less than 0.002%, the manufacturing cost will increase dramatically and the economic efficiency will be impaired.
The lower limit is 0.002%, and if it exceeds 0.030%, the workability deteriorates, so 0.030% is the upper limit. If S is reduced to less than 0.002%, the manufacturing cost will be drastically increased and the economy will be impaired, so 0.02% is the lower limit, and if it exceeds 0.050%, the workability deteriorates, so 0.050% is the upper limit.

When Al is less than 0.002%, deoxidation is insufficient and blowholes are generated in the steel, impairing cleanliness as a steel sheet, cracks at the time of pressing, and surface defects. The lower limit is set, and if 0.100% is exceeded, the workability will deteriorate, so 0.100% is set as the upper limit. N is preferably as small as possible, but if it is less than 0.0002%, the manufacturing cost will increase significantly, so 0.00
If the upper limit is 02% and the upper limit is 0.0100%, the age-hardenability is high and the workability is deteriorated. Therefore, the upper limit is 0.0100%.

Next, the reason for limiting the components of the surface layer will be described. Cu is an essential element for ensuring corrosion resistance. Cu, in the presence of P, forms an amorphous rust layer at the boundary between the rust layer having a normal composition and the base iron when rust is generated on the surface of the steel sheet, and has an action of hindering the progress of rust. Improves the corrosion resistance of steel sheets. If it is less than 0.15%, the effect of improving the corrosion resistance cannot be sufficiently exerted, so 0.15% is made the lower limit. Also, 2.
If the content exceeds 0%, the ductility deteriorates and cracks particularly occur during the overhanging process, so 2.0% is made the upper limit.

P is an element which, together with Cu, helps improve the corrosion resistance. Since it is contained in general steel, it is also contained in the components of the inner layer, but in the present invention, the content of the inner layer is made as small as possible from the viewpoint of ensuring workability, and the surface layer contains the amount necessary for ensuring corrosion resistance. If the amount contained in the surface layer is less than 0.030%, the effect of improving the corrosion resistance is lost, so the lower limit is 0.030%, and if it exceeds 0.150%, cracking tends to occur during processing, so 0.150% is the upper limit.

Ni is added to prevent surface cracks that occur when Cu is added. When producing steel containing Cu, if the heating temperature is high in the heating stage before hot rolling, cracks called net cracks occur on the surface. It is added in order to prevent this, but it is desirable that the added amount is about 0.1 to 2.0 times the Cu content. Therefore, 0.08 to 2.0% is included. In a manufacturing facility that can lower the heating temperature (for example, when heating at 1050 ° C or lower is possible), there is no surface cracking, so N to prevent surface cracking
i need not be added.

In the present invention, the above-mentioned surface layer component is provided,
The surface layer composition is adjusted in the continuous casting stage. The method is to vertically inject the steel having the above-mentioned inner layer component into the mold for continuous casting together with the gas, and to apply a static magnetic field to the entire width in the width direction in the mold above this molten steel injection position to raise the steel. The flow is slowed down, Cu and P or Cu and P and N are added to the steel above the position where the magnetic field is applied, so that the molten steel in the upper portion becomes the above-mentioned surface layer component by stirring the injected gas, It is a method of forming a slab having the above-mentioned composition by drawing the mold.

This method will be described with reference to the drawings. FIG.
In FIG. 2, in the continuous casting mold 3 including the long-side mold 1 and the short-side mold 2, the lower end open type immersion nozzle 4 is arranged so as to be connected to a tundish (not shown). 3, a static magnetic field (N pole) 5 and a static magnetic field (S) for imparting a static magnetic field in the mold 3 are provided above the injection port 6 at the lower end of the immersion nozzle 4, which is the molten steel injection position.
5a is arranged so as to extend over the entire width of the long side mold 1, that is, the width of the slab 7.

At the time of casting, the molten steel 11 having the above-mentioned inner layer components is injected into the mold 3 by the immersion nozzle 4, and at the same time, gas is blown from the gas blowing port 8 of the immersion nozzle 4. On the other hand, above the injection port 6 at the lower end of the immersion nozzle 4 which is the injection position of the molten steel 11, the mold 3 is formed by the static magnetic fields 5 and 5a arranged so as to cover the entire width of the long side mold 1 in the width direction.
An element 9 (Cu and P or Cu and P and Ni) to be added to the molten steel 11 to be a surface layer by imparting a static magnetic field into the molten steel injected into the inside and decelerating the upward flow of the molten steel by this static magnetic field Is added to make the upper molten steel in the mold 3 an alloy containing the above-mentioned surface layer components.

Then, this is continuously cast and drawn downward as a slab 7, and as shown in FIG. 3, the above-mentioned surface layer component is added only to the surface layer 10a, and the inner layer 11a forms the multi-layer slab 7 which is the inner layer component. To cast.

When the molten steel 11 injected from the immersion nozzle 4 into the mold 3 is injected vertically together with the gas from the injection port 6 of the immersion nozzle 4 in the vertical (downward) direction, the molten steel 11 shows an arrow in the mold 3. The reversed upflow 12 moves upward, and the static magnetic fields 5 and 5a above the injection port 6 apply the static magnetic field.

When a static magnetic field is applied in this way, the molten steel 11
Although the upflow 12 which has been reversed is rapidly decelerated, the molten steel 11 is decelerated and moves to the upper part of the static magnetic field 5, 5a, where the element 9 to be added to the molten steel 11 is added and the molten alloy steel is added. It becomes 10.

On the other hand, the gas injected vertically from the injection port 6 of the dipping nozzle 4 together with the molten steel 11 becomes fine bubbles 13 that are finely dispersed and rise in the entire area of the molten steel. The molten element 9 is stirred to form a homogenized molten alloy steel 10.

Then, by pulling downward as a cast piece 7 from the mold 3, the molten alloy steel 1 above the static magnetic field 5, 5a
When the surface of the molten steel 11 is cooled and solidified, and is drawn and moved below the static magnetic fields 5 and 5a, the steel is solidified by the molten steel 11 to which the additional element is not added and is used as the inner layer 11a, and only the surface is drawn and moved. At the same time, the solidified layer of the molten alloy steel 10 becomes the multi-layer cast piece 7 in which the surface layer 10a of the alloy steel in which the solidified layer is gradually expanded is formed.

In this way, by injecting the molten steel 11 vertically downward from the injection port 6 of the immersion nozzle 4 together with the gas,
After the injection flow of the molten steel 11 reaches the lower side, it is reversed by the buoyancy of the gas and rises to the ascending flow 12, but as the flow velocity at this time rises, it becomes gentle in the vicinity of the static magnetic fields 5 and 5a and is immersed. Above the injection port 6 of the nozzle 4, the upward magnetic field is rapidly suppressed by applying the static magnetic field by the static magnetic fields 5 and 5a.

Therefore, the alloy steel 10 above the static magnetic fields 5 and 5a is not greatly disturbed, and the static molten magnetic field is blocked by the molten steel 11 in the lower part of the mold 3 and the molten steel confidence increases. The molten steel 10 is not mixed by the flow 12, and the alloy steel 10a is reliably and stably formed in the inner layer 11a of the steel.
It is possible to obtain the multilayer cast slab 7 formed on the surface of the. However, it is only necessary to form an upward flow that is the reverse of the injection flow, so the injection port of the immersion nozzle may be the one-hole type shown in FIG. 1 or the ordinary two-hole type. The layer thickness of the surface layer 10a can be accurately controlled by the casting speed, that is, the drawing speed and the installation position of the static magnetic field.

In the present invention, the thickness of the surface layer is the total thickness t.
It is 0.015 to 0.15t. The reason for this is that the surface layer thickness is 0.015
If it is less than t, sufficient Cu and P cannot be supplied to form an amorphous rust layer at the boundary between the normal rust layer and the base steel.
If the upper limit is 5 t and the upper limit is more than 0.15 t, the proportion of the layer containing a high alloy component increases, and the workability as the object of the present invention is impaired. Therefore, the upper limit is 0.15 t.

Specifically, when a static magnetic field is installed in the mold, for example, the drawing speed is 0.3 to 2.0 m / min and the surface layer thickness is 10 to 3
It can be controlled to 0 mm, and the lower the drawing speed, the thicker the surface layer, and the higher the speed, the thinner the surface layer. That is, at low speeds, that much molten alloy steel 1
The surface of No. 0 is in contact with the mold 3 for a long time, and accordingly, the time for cooling becomes long, and the thickness of the surface layer 10a serving as a solidification layer becomes thicker. This is because the contact time of the surface of the mold with the mold 3 is shortened, the cooling time is shortened, and the thickness of the surface layer 10a serving as the solidified layer is reduced.

The slab obtained as described above is made into a steel plate according to a conventional method for manufacturing a steel plate. First, the slab is continuously cast, cooled directly or once to an appropriate temperature, and then heated in a heating furnace. This heating temperature is 900 ° C, which enables hot rolling.
It is desirable to set the temperature to about 1300 ° C. The hot rolling carried out after the heating may be carried out below the A3 transformation point, but it is preferably not less than the A3 transformation point because sufficient workability cannot be obtained.

After hot rolling, the hot rolled steel sheet is wound at an appropriate temperature. Although the product can be used as it is, if a steel plate having a thinner plate thickness or higher workability is required, after this, after performing descaling treatment such as pickling, 5
Cold rolling of 0% or more is performed, and then annealing is performed at a temperature of a recrystallization temperature or higher to obtain a cold rolled steel sheet. After cold rolling, skin pass rolling is performed to obtain a product.

Further, hot-rolled steel sheet or cold-rolled steel sheet obtained by the present invention may be subjected to hot dip coating and / or electroplating on one side or both sides for the purpose of improving paintability, weldability and the like. It does not depart from the invention. Further, it is of course possible to perform various treatments on the multilayer steel sheet of the present invention, and for example, even if chromate treatment, phosphate treatment, resin coating treatment and the like are performed, it does not depart from the scope of the present invention,
Various types of processing can be performed depending on the additionally required characteristics.

[0029]

【Example】

Example 1 Molten steel having the components listed in Table 1 was prepared as an inner layer component.
Then, it was cast by the following method to obtain a steel plate.

[Table 1] 1) Mold size 245 mm (short side) x 1200 mm (long side) Mold height 900 mm 2) Static magnetic field position (electromagnetic coil center position) molten steel surface 430 mm below 3) Immersion nozzle injection port position 50 mm below static magnetic field position 4) Immersion Nozzle inlet diameter φ90mm

Into such a continuous casting machine, molten steel having the inner layer components shown in Table 1 was injected from the dipping nozzle into the mold together with 3.0 l / min of Ar gas, while a pure Cu wire was introduced from the static magnetic field into the molten steel above. And a cored wire having FeP as a core material were added, and a casting was performed at a drawing speed of 1.3 m / min while applying a static magnetic field of 5000 gauss.

By controlling the addition rates of the pure Cu wire and the FeP wire, the Cu content in the surface layer portion was reduced to 0.
20 to 1.55%, P content 0.050 to 0.12
A multi-layer slab having a uniformly formed surface layer portion having a surface layer portion thickness of 12 to 13 mm was obtained at 5%.

Next, the slab was heated to 1050 ° C., then hot rolled at 890 ° C. and wound at 700 ° C. to obtain a hot rolled steel sheet having a plate thickness of 4.8 mm. The steel sheet was pickled to remove the scale and then cold rolled to obtain a 1.0 mm steel sheet. Then, continuous annealing was performed at 800 ° C. for 60 seconds, and
8% skin pass rolling was performed to obtain a cold rolled steel sheet. Further, as a comparative example, a steel plate having a component consisting of only the inner layer component was manufactured by the same method except for the continuous casting method.

Corrosion resistance of these steel sheets, salt spray and wet,
It was evaluated by a corrosion test in which drying was repeated. In particular,
Immersion type phosphate treatment on steel plate (BTL30 manufactured by Nippon Parker
80), followed by cationic electrodeposition coating (Nippon Paint Co., Powertop D-30, 20μ coating), cross-cut reaching the substrate, 5% NaCl salt spray at 35 ° C for 6 hours, → forced drying. Erosion depth of the cross-cut portion was evaluated in mm unit when 80 cycles of a corrosion acceleration test with 70 ° C., RH 60% for 4 hours, → cooling −20 ° C., 4 hours as one cycle.

The workability was evaluated by using a circular steel plate punched to a diameter of 80 mm, performing deep-drawing on a flat-bottomed cylinder at various drawing ratios, and determining the limiting drawing ratio. Table 2 shows the evaluation results. From this table, it is understood that the steel of the present invention has a smaller amount of erosion in the cross-cut portion and a limiting drawing ratio of more than 1.8, and has excellent corrosion resistance and workability, as compared with the comparative examples.

[0035]

[Table 2]

Example 2 Molten steel having the component A in Table 1 as an inner layer component was prepared. Then, it was cast by the following method to obtain a steel plate. 1) Mold size 245 mm (short side) x 1500 mm (long side) Mold height 900 mm 2) Static magnetic field position (electromagnetic coil center position) Molten steel surface 430 mm below 3) Immersion nozzle injection port position 50 mm below static magnetic field position 4) Immersion Nozzle inlet diameter φ90mm

Component A in Table 1 was added to such a continuous casting apparatus.
Molten steel is injected into the mold with 3.0 l / min of Ar gas from the dipping nozzle, while a pure Cu wire and a cored wire having FeP as a core material and a pure Ni wire are added from the static magnetic field to the upper molten steel. Together with 5000
Extraction speed 2.5m / while applying a static magnetic field of Gauss
Min, 1.3 m / min, and 0.3 m / min.
By controlling the addition rate of the pure Cu wire, the FeP wire, and the pure Ni wire, a slab having the components shown in Table 3 as the surface layer component was obtained.

[Table 3]

Then, the cast slab was heated to 1250 ° C., then hot rolled at 890 ° C. and wound at 700 ° C. to obtain a hot rolled steel sheet having a plate thickness of 4.0 mm. The steel sheet was pickled to remove scale, and then cold rolled to give a 0.8 mm steel sheet. Then, continuous annealing was performed at 800 ° C. for 60 seconds, and
8% skin pass rolling was performed to obtain a cold rolled steel sheet.

The corrosion resistance of these steel sheets was evaluated by salt spray and wetting,
It was evaluated by a corrosion test in which drying was repeated. In particular,
Immersion type phosphate treatment on steel plate (BTL30 manufactured by Nippon Parker
80), followed by cation electrodeposition coating (Nippon Paint Co., Powertop D-30, 20μ coating), cross-cut reaching the substrate, 5% NaCl salt spray at 35 ° C for 6 hours, → forced drying. Erosion depth of the cross-cut part was evaluated in mm unit when 80 cycles of corrosion acceleration test with 70 ° C, RH 60%, 4 hours, → cooling -20 ° C, 4 hours as one cycle.

The workability was evaluated by using a circular steel plate punched out to a diameter of 80 mm, subjecting a flat-bottomed cylinder to deep drawing at various drawing ratios, and determining the limiting drawing ratio. Table 4 shows the evaluation results. From this table, it is understood that the steel of the present invention has a smaller amount of erosion in the cross-cut portion and a limiting drawing ratio of more than 1.8, and has excellent corrosion resistance and workability, as compared with the comparative examples.

[0041]

[Table 4]

[0042]

According to the present invention, a steel sheet containing a large amount of Cu and P, which are elements having excellent corrosion resistance, on the surface of the steel sheet, and containing a small amount of Cu and P, which deteriorate workability, is provided inside the steel sheet. It is possible to obtain a steel sheet having both corrosion resistance and workability. Further, in the method for producing a steel sheet, the control of the surface layer thickness can be accurately controlled by the drawing speed of the cast slab and the static magnetic field installation position, so that a multi-layered steel sheet with stable surface layer thickness and excellent workability is produced. can do.

[Brief description of drawings]

FIG. 1 is a side view illustrating a casting method of the present invention.

FIG. 2 is a plan view illustrating a casting method of the present invention.

FIG. 3 is a cross-sectional view of a multi-layer cast piece cast according to the present invention.

[Explanation of symbols]

 3 Mold 4 Immersion Nozzle 5 Static Magnetic Field (N Pole) 5a Static Magnetic Field (S Pole) 6 Immersion Nozzle Inlet 8 Gas Inlet 9 Wire Containing Additional Element 10a Surface Layer (Solidified Layer of Molten Steel Having Surface Layer Components) 11 Molten Steel 11a Inner layer (solidified layer of molten steel)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/06 38/16

Claims (3)

[Claims] 1. In a multi-layer steel sheet having different components in the surface layer and the inner layer, C: 0.01 to 0.20%, Si: 0.002 to 1.00%, Mn: 0.02 to 3.0%, and P: 0.002 to 0.030 by weight ratio as the inner layer components. %, S: 0.002-0.050%, Al: 0.002-0.100%, N: 0.0002-0.0100%, balance: Fe and unavoidable impurities. When the plate thickness is t, one side is 0.
The surface layer component of the portion of 015t to 0.15t is C, Si, Mn,
S, Al and N are the same as the components of the inner layer, and P: 0.030 to 0.150%, Cu: 0.15 to 2.0%, balance: Fe and a multi-layer steel sheet containing inevitable impurities and excellent in corrosion resistance and workability. . 2. A multi-layered steel sheet having different components in the surface layer and the inner layer, having the inner layer component according to claim 1 as the inner layer component and having a plate thickness t, 0.01 from the front surface to the back surface.
A multi-layered steel sheet having excellent corrosion resistance and workability, wherein the surface layer component in the 5t to 0.15t portion contains Ni: 0.08 to 2.0% in addition to the surface layer component according to claim 1. 3. A molten steel having the inner layer component according to claim 1 is injected vertically downward or obliquely downward together with a gas into a mold for continuous casting, and a static magnetic field is applied to the entire width direction in the mold above the position where the molten steel is injected. Applied to slow down the ascending flow of the steel, and Cu and P or Cu to the steel above the position where the magnetic field is applied.
And P and Ni are added, and the molten steel in the upper part is made to be the surface layer component according to claim 1 or 2 by stirring the injected gas, and the component composition according to claim 1 or 2 is obtained by mold drawing. And then subjected to hot rolling, descaling, cold rolling, and annealing according to a conventional method for producing a steel sheet, and having the components according to claim 1 or claim 2 in the inner layer,
When the plate thickness is t, the surface layer component of 0.015 t to 0.15 t from the front and back faces is a cold-rolled steel sheet containing the surface layer component according to claim 1 or 2, which is excellent in corrosion resistance and workability. Manufacturing method of multi-layer steel sheet.