JPH08127841A - Multiply-layered steel sheet excellent in deep drawability, dent resistance and fatigue property and production thereof - Google Patents

Abstract

PURPOSE: To produce a multiply-layered steel sheet excellent in deep drawability, dent resistance and fatigue properties by constituting it of an internal layer contg. specified amounts of C, Si, Mn, P, S, Al, N, Ti and Nb and a surface layer contg. Cu as well. CONSTITUTION: The components in the internal layer of a multiply layered steel sheet are constituted of, by weight, 0.0002 to 0.0090% C, 0.002 to 1.0% Si, 0.02 to 3.0% Mn, 0.002 to 0.150% P, 0.002 to 0.050% S, 0.002 to 0.100% Al, 0.0002 to 0.0100% N and one or more kinds of 0.005 to 0.10% Ti and 0.005 to 0.15% Nb as carbon nitride forming components, furthermore of 0.0003 to 0.0050% B if necessary, and the balance Fe with inevitable impurities. The components in the surface layer at the part of 0.015 t to 0.15 t ((t) denotes sheet thickness) per side of the surface and rear faces are constituted of the ones obtd. by adding the same components with 0.50 to 3.0$ Cu and furthermore with 0.05 to 4.0% Ni as well according to necessary. This multiply- layered steel sheet can be obtd. by adding Cu or the like to the surface of the molten steel having the same internal layer components and executing continuous casting while a magnetic field is applied to a mold.

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 dent resistance of a steel sheet, and for example, a steel sheet having a multi-layer structure as disclosed in Japanese Patent Laid-Open No. 4-191330 is known. However, such a steel has a C surface layer as a technique for improving dent resistance.
By increasing the content and strengthening only the surface layer to obtain dent resistance, the diffusion of C occurs during annealing, and since C penetrates into the inner layer, the effect of solid solution C causes a high r. It had the drawback that no value could be obtained. Therefore, such a steel sheet cannot be applied to parts that require particularly high deep drawability. Further, in general, an ultra-low carbon steel having a good deep drawability has a drawback that its fatigue property is inferior to that of a low carbon steel.

[0003]

SUMMARY OF THE INVENTION According to the present invention, when the conventional ultra-low carbon steel attempts to improve the dent resistance, the deep drawability becomes inferior and the fatigue property is inferior. A multi-layer steel sheet and a method for manufacturing the same have been made to solve the above problems.

[0004]

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: 0.0002 to 0.0090%, Si: 0.00 as the inner layer component.
2 to 1.00%, Mn: 0.02 to 3.0%, P: 0.002 to 0.150
%, S: 0.002-0.050%, Al: 0.002-0.100%,
N: 0.0002 to 0.0100%, Ti: 0.005 to 0.050% and N
b: 0.005 to 0.050% of one or two kinds, the balance: Fe and inevitable impurities (the above component system is (A)), or B: 0.0003 to the component system (A).
If 0.0050% is included and the plate thickness is t, 0.01 from the front and back
The surface layer components in the 5t to 0.15t portion are C, Si, Mn,
P, S, Al, N, Ti, Nb, B, Fe and unavoidable impurities are the same as those of the inner layer component, and Cu: 0.
50-3.0% or Cu: 0.50-3.0% and Ni: 0.05
It is a multi-layer steel sheet containing up to 4.0% with excellent deep drawability, dent resistance and fatigue characteristics.

Further, according to the present invention, the steel having the inner layer component is injected vertically downward or obliquely downward into a mold for continuous casting, and a static magnetic field is applied to the entire width direction in the mold above the molten steel injection position. To slow down the ascending flow of the steel to the steel above the position where the magnetic field is applied.
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, and having the above-mentioned inner layer components in the inner layer, and the plate thickness is t, 0.015 t to 0.15 t per one surface from the front and back surfaces are cold components containing the above surface layer components. It is a method for producing a multi-layer steel sheet excellent in deep drawability, dent resistance, and fatigue characteristics, which is characterized in that it is formed of a rolled steel sheet.

A feature of the present invention is that the dent resistance is improved by making Cu, whose strength is increased without affecting the deep drawability, limited to only the surface portion of the steel sheet which is particularly effective for the dent resistance. To secure. Further, the strengthening by Cu is characterized in that it is excellent in fatigue strength, and by making it exist only on the surface of the steel sheet, it is possible to obtain a very high bending fatigue strength as compared with the strength of the entire steel sheet.

The reasons for limiting the components of the inner layer will be described below.
If C is less than 0.0002%, the cost for decarburization becomes extremely high and it is not economical, so 0.0002% is made the lower limit. Further, if it exceeds 0.0090%, even if a carbide forming element is added, the amount of carbide increases and the workability deteriorates.
% Is the upper limit. If Si is reduced to less than 0.002%, the manufacturing cost is significantly increased and the economic efficiency is 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 3.
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 0.150%, the workability deteriorates, so 0.150% is the upper limit.

If S is reduced to less than 0.002%, the manufacturing cost is dramatically increased and the economic efficiency is impaired.
% Is the lower limit, and if it exceeds 0.050%, the workability deteriorates, so 0.050% is the upper limit. Al is less than 0.002%,
Due to lack of deoxidation, blowholes will occur in the steel,
0.002% is the lower limit because it impairs cleanliness as a steel plate and causes cracks and surface defects during pressing.
If it exceeds 0.10%, the workability will deteriorate, so 0.100%
Is the upper limit.

N is preferably as small as possible, but 0.0002
If it is less than 0.1%, the production cost is significantly increased, so 0.0002% is the lower limit, and if it exceeds 0.0100%, age hardening becomes high and the workability deteriorates, so 0.0100% is made the upper limit. Carbon and nitride forming element groups, Ti and Nb, are added in order to precipitate and fix C and N in the steel and reduce solid solution C and solid solution N that deteriorate workability.

If the content of Ti is less than 0.005%, the amount of carbonitrides formed is insufficient, and the amount of solid solution C and solid solution N in the steel increases to deteriorate the workability. Therefore, 0.005% is the lower limit, and 0.10% If the content exceeds 0.10%, the amount of solid solution Ti present in the steel increases, and in this case as well, workability deteriorates.
Is the upper limit.

If the amount of Nb is less than 0.005%, the amount of carbonitrides formed is insufficient, and the amount of solid solution C and solid solution N in the steel increases to deteriorate the workability. Therefore, 0.005% is the lower limit, and 0.15% If the content exceeds 0.15%, the amount of solute Nb present in the steel increases, and in this case too, the workability deteriorates.
Is the upper limit.

When ultra-low carbon steel is used for deep drawing, B may cause secondary work cracking, so B is added to prevent this. The addition of B dramatically improves the resistance to secondary work cracking. If the addition amount is less than 0.0003%, the effect of preventing secondary work cracking is lost, so 0.0003% is the lower limit, and
If the content exceeds 50%, the recrystallization temperature rises and annealing becomes difficult, or hot cracking occurs during hot rolling, so 0.0050% is made the upper limit.

Next, the reason for limiting the components of the surface layer will be described. Cu is an essential element for ensuring the dent resistance. C
u increases the strength of the steel sheet by solid solution strengthening. 0.50%
If it is less than 0.5%, the effect of improving dent resistance cannot be sufficiently exhibited, so 0.50% is made the lower limit. Further, if it is contained in an amount of 3.0% or more, the ductility deteriorates, and cracks particularly occur during the overhanging process, so 3.0% is made 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 to prevent this, but the amount of addition must be about 0.1 to 1.3 times the Cu content. For this reason, 0.05 to 3.0% is contained. Manufacturing equipment that can lower the heating temperature (for example, when heating below 1050 ° C is possible)
Then, it is not necessary to add it because there is no cracking on the surface.

In the present invention, the above-mentioned surface layer component is provided,
The surface layer component is adjusted during continuous casting. The method, the steel having the above-mentioned inner layer component is injected vertically downward or obliquely downward with gas into a mold for continuous casting, and a static magnetic field is applied to the entire width direction in the mold above the molten steel injection position, The ascending flow of steel is decelerated, Cu or Cu and Ni are added to the steel above the position where the magnetic field is applied, and the molten steel in the upper part becomes the above-mentioned surface layer component by stirring the injected gas. This is a method of forming a slab having the above-mentioned composition by drawing.

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 dipping nozzle 4 at the pouring position of the molten steel 11, the static magnetic fields 5 and 5a arranged so as to cover the entire width of the long side mold 1 into the mold 3. A static magnetic field is applied to the injected molten steel, and the element 9 (Cu or Cu and Ni) to be added to the molten steel 11 to be the surface layer is added while decelerating the upward flow of the molten steel by this static magnetic field The molten steel in the upper part of 3 is an alloy containing the above-mentioned surface layer components.

Then, this is continuously cast and drawn out 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 a multi-layer slab 7 which is the inner layer component. To cast.

The molten steel 11 injected from the immersion nozzle 4 into the mold 3 is shown by an arrow in the mold 3 when injected vertically (downward) from the injection port 6 of the immersion nozzle 4 together with the gas. It becomes a reverse reversal upward flow 12 and moves upward,
Here, a static magnetic field is applied by the static magnetic fields 5 and 5 a located above the injection port 6. When the static magnetic field is applied in this manner, the reversed upward flow of the molten steel 11 is rapidly decelerated, but the decelerated speed causes the molten steel 11 to move to the upper part of the static magnetic fields 5 and 5a. The element 9 to be added is added to form molten alloy steel 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 is finely dispersed as bubbles 13 and rises in the entire area of the molten steel, and is added above the added injection port 6. The molten element 9 is stirred to form a homogenized molten alloy steel 10. And from the mold 3 to the slab 7
The surface of the molten alloy steel 10 above the static magnetic fields 5 and 5a is cooled and solidified by pulling downward as the solid magnetic field, and no additional element is added when the molten steel 10 is drawn and moved below the static magnetic fields 5 and 5a. The steel obtained by the solidification of the molten steel 11 becomes the inner layer 11a, and only the surface becomes the multi-layer cast piece 7 in which the surface layer 10a of the alloy steel in which the solidified layer of the molten alloy 10 gradually expands is formed with the movement.

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 molten steel 11 below the mold 3 has a static magnetic field blocking action and an increase in molten steel confidence. 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, the injection port of the immersion nozzle may be a one-hole type as shown in FIG. 1 or an 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 0.03t.
If it is less than 0.15 t, the strength cannot be sufficiently increased to secure the dent resistance. Therefore, if the lower limit is 0.015 t, and if it exceeds 0.15 t, the ratio of the layer containing high alloy components becomes high, and the strength of the entire steel sheet is Since it becomes higher and the deep drawability which is the object of the present invention is impaired, the upper limit is 0.15 t.

Specifically, when a static magnetic field is set in the mold, 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 mixing of the components at the boundary between the inner layer and the surface layer does not change the properties of the steel sheet of the present invention, so that the mixing of the components of the surface layer and the inner layer is allowed.

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. It is desirable that the heating temperature is about 900 ° C. to 1300 ° C. at which hot rolling can be performed. The hot rolling performed after heating may be hot rolled below the A3 transformation point, but it is preferably at least 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, a hot-rolled steel sheet obtained by the method of the present invention,
It is possible to perform hot dipping and / or electroplating on one side or both sides of the cold rolled steel sheet for the purpose of improving corrosion resistance, paintability, and weldability, which does not depart from the present invention.

It is of course possible to add various treatments to the multi-layer steel sheet of the present invention, for example, chromate treatment, phosphate treatment, treatment for improving phosphate treatment, and lubricity improving treatment. Even if the weldability improving treatment and the resin coating treatment are performed, it does not depart from the scope of the present invention, and various treatments can be additionally performed depending on the required characteristics.

[0031]

【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 apparatus, the molten steel shown in Table 1 was injected into the mold from an immersion nozzle together with 3.0 l / min of Ar gas, while a pure Cu wire was added into the molten steel from the static magnetic field. In addition, while applying a static magnetic field of 5000 gauss, casting was performed at a drawing speed of 1.3 m / min. Pure C
By controlling the addition rate of the u-wire, the Cu content of the surface layer part is 0.54 to 1.85%, and the thickness of the surface layer part is 12 to 13 mm. Obtained.

Then, the slab was heated to 1050 ° C., then hot rolled at 910 ° 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. In addition, as comparative examples, a steel sheet having a small amount of Cu in the outer layer and a steel sheet having no Cu added to the outer layer were manufactured.

The dent resistance, fatigue property and deep drawability of these steel sheets were evaluated. For dent resistance, use a steel plate with a width of 300 mm.
Molded into a semi-cylindrical panel with a radius of curvature of 50 mm, press a spherical steel indenter with a radius of 10 mm against the center with a load of 20 kg, and measure the depth of the depression that occurred at that time.
Evaluation was made based on this value.

The fatigue characteristics were evaluated by obtaining the strength at the fatigue limit of 10 ⁷ times by the double swing plane bending fatigue test, and using the ratio of this value to the tensile strength measured by static tension. The deep drawability was evaluated by using a circular steel plate punched out to a diameter of 80 mm, performing a flat-bottomed cylinder deep draw at various draw ratios, and determining the limit draw ratio.

The evaluation results are shown in Table 2. From this table, the steel of the present invention has a high dent resistance with a dent depth of 0.4 mm or less, even though the deep drawability shows a high value of 2.0 or more, as compared with the comparative example, and fatigue It can be seen that it has high fatigue characteristics with a limit ratio of 0.55 or more.

[0037]

[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) × 1500 mm (long side) Mold height 900 mm 2) Static magnetic field position (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 Injection port diameter 90mm

Into such a continuous casting apparatus, the molten steel shown in Table 1 was injected from the dipping nozzle into the mold together with 3.0 l / min of Ar gas. Withdrawing speed 2.5m / while adding Ni wire and applying static magnetic field of 5000 gauss
Min, 1.3 m / min, and 0.3 m / min.
By controlling the addition rate of the pure Cu wire and the pure Ni wire, a cast piece 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 910 ° 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 characteristics of these steel sheets were evaluated in the same manner as in Example 1. Table 4 shows the evaluation results. From this table, it can be seen that the steels of the present invention have high dent resistance and high fatigue properties, even though they show the same high values of deep drawability as compared with the comparative examples.

[0042]

[Table 4]

[0043]

According to the present invention, a steel sheet excellent in dent resistance and fatigue characteristics can be obtained without impairing the deep drawability. According to the present invention, the surface of the steel sheet contains a large amount of Cu, which is an element that increases the strength without impairing the deep drawability, and the inside of the steel sheet is an extremely low carbon steel with a particularly good deep drawability, so that the dent resistance is improved. It is possible to provide a steel sheet having excellent fatigue characteristics. Further, in the method for producing the steel sheet, the surface layer thickness can be accurately controlled by the withdrawal speed of the slab and the static magnetic field installation position, so that the multilayered layer has a stable surface layer thickness and is excellent in dent resistance and deep drawability. Steel sheets can be manufactured.

[Brief description of drawings]

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

FIG. 2 is a plan view illustrating a casting process 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 Office reference number FI technical display location B32B 15/01 A C22C 38/14 38/16

Claims (4)

[Claims] 1. In a multi-layered steel sheet having different components in the surface layer and the inner layer, C: 0.0002 to 0.0090%, Si: 0.002 to 1.0%, Mn: 0.02 to 3.0%, P: 0.002 to 0.150 by weight% as an inner layer component. %, S: 0.002-0.050%, Al: 0.002-0.100%, N: 0.0002-0.0100%, Ti and Nb of carbonitride forming element group Ti: 0.005-0.10%, Nb: 0.005-0.15% Or 2 types, the balance: Fe and unavoidable impurities are contained, and the plate thickness is t.
The surface layer component of the portion of 015t to 0.15t is C, Si, Mn,
P, S, Al, N, Ti, Nb, Fe and unavoidable impurities are the same as the components of the inner layer, and Cu: 0.50 to 3.0% is contained, which is excellent in deep drawability, dent resistance and fatigue characteristics. Multi-layer steel sheet. 2. A multi-layer steel sheet having different components in the surface layer and the inner layer, which has the component structure according to claim 1, and as an inner layer component and a surface layer component, in addition to the component according to claim 1, B: 0.0003 to 0.0050. %, A multi-layer steel sheet with excellent deep drawability, dent resistance, and fatigue properties. 3. A multi-layer steel sheet having different components in the surface layer and the inner layer, having the inner layer component according to claim 1 or 2 as the inner layer component and having a plate thickness t, 0.015 t to 0.15 per side from the front and back surfaces. The surface layer component in the portion t is claim 1 or 2.
In addition to the listed surface layer components, a multi-layer steel sheet containing Ni: 0.05 to 4.0% and having excellent deep drawability, dent resistance, and fatigue characteristics. 4. The steel having the inner layer component according to claim 1 or 2 is injected vertically downward or obliquely downward together with a gas into a continuous casting mold, and statically filled over the entire width direction in the mold above the molten steel injection position. A magnetic field is applied to slow down the ascending flow of the steel, and Cu or C is added to the steel above the position where the magnetic field is applied.
u and Ni are added so that the molten steel in the upper portion becomes the surface layer component according to claim 1 or 3 by stirring the injected gas.
A slab having the composition according to claim 1, 2 or 3 is obtained by drawing the mold, and then hot rolling, descaling treatment, cold rolling and annealing are performed based on a conventional method for producing a steel sheet. Or, having the components described in 2 and having a plate thickness of t, a cold rolled steel sheet containing 0.015 t to 0.15 t of surface layer components from one surface to the other surface is the cold rolled steel sheet containing the surface layer components of claim 1, 2 or 3. A method for producing a multi-layer steel sheet having excellent deep drawability, dent resistance, and fatigue characteristics.