JPH08283903A - Multi-layered steel sheet excellent in brazing crack resistance and its production - Google Patents

Abstract

PURPOSE: To produce a multi-layered steel sheet excellent in brazing crack resistance without deteriorating workability by providing a surface layer containing specific amounts of B to a specific thickness on an inner layer com posed of dead-soft steel having a specific composition containing carbonitride- forming elements, such as Ti and Nb. CONSTITUTION: In a multi-layered steel sheet consisting of surface layer and inner layer dissimilar in composition, the inner layer is composed of a dead-soft steel excellent in workability, which has a composition containing, as essential components, 0.0002-0.0080%, by weight, C, 0.002-1.00% Si, 0.02-3.0% Mn, 0.002-0.150% P, 0.002-0.050% S, 0.002-0.100% Al, and 0.0002-0.0100% N, further containing either or both of 0.003-0.100% Ti and 0.003-0.100% Nb as carbonitride- forming elements, and having the balance Fe with inevitable impurities. Further, as a surface layer to be formed on the inner layer, a layer having a composition prepared by further adding 0.0003-0.0050% B to the composition of the inner layer to improve brazing crack resistance is formed, and moreover, its thickness is regulated to 0.015-0.15t [where (t) means sheet thickness] per side from the surface and the rear surface, respectively.

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 formability of a steel sheet, and an ultra-low carbon steel sheet such as that disclosed in Japanese Patent Publication No. 44-18066 is known. However, in such an ultra-low carbon steel sheet, when the steel purity is increased in order to further improve the workability, when a steel sheet is joined by brazing or the like, a metal having a low melting point such as Cu is formed in the grain boundary. It is easy to penetrate and causes so-called solder brittleness, and was not suitable for applications such as brazing.

[0003]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the problem that the conventional ultra-low carbon steel becomes inferior in brazing cracking resistance when it is highly purified to improve workability. It is a thing.

[0004]

The gist of the present invention is as follows. (1) In a multi-layer steel sheet having different components in the surface layer and the inner layer, C: 0.0002-in weight% as the inner layer component
0.0080%, Si: 0.002-1.00%, M
n: 0.02-3.0%, P: 0.002-0.150
%, S: 0.002-0.050%, Al: 0.002
~ 0.100%, N: 0.0002-0.0100%,
In addition to the basic components, Ti: 0.
003 to 0.100 and Nb: 0.003 to 0.100%
1 or 2 of the above, the balance: Fe and unavoidable impurities are contained, and the plate thickness is 0.01
A multi-layer steel sheet having excellent brazing cracking resistance, which contains B: 0.0003 to 0.0050% as a surface layer component of a portion of 5t to 0.15t, in addition to the inner layer component.

(2) Molten steel having the above-mentioned 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 of the mold above the position where the molten steel is injected. The rising flow of the molten steel is decelerated, B is added to the molten steel above the position where the magnetic field is applied, the molten steel in the upper portion is made to be the above-mentioned surface layer component by stirring the injected gas, and by the mold drawing, Without a slab having the above-mentioned composition, then, based on the ordinary method of steel plate production,
When hot-rolled, or hot-rolled, descaled, cold-rolled, and annealed to have the above-mentioned inner layer components in the inner layer and the plate thickness is t, 0.015 t to 0. 15t
Is a hot-rolled steel sheet or cold-rolled steel sheet containing the surface layer component described above, and is a method for producing a multilayer steel sheet having excellent brazing crack resistance.

[0006]

FUNCTION B is an element that improves brazing crack resistance. However, if a large amount of this element is contained, the workability is deteriorated and the recrystallization temperature rises, which makes the manufacturing difficult. The feature of the present invention is that B is present only in the surface region particularly effective for brazing cracking resistance, and the inner layer remains extremely low carbon steel, and the brazing cracking resistance is maintained without impairing workability. To secure.

The reasons for limiting the components of the inner layer will be described below.
If C is less than 0.0002%, the production cost for decarburization will increase dramatically and the economic efficiency will be impaired.
The lower limit is 2%. Further, if it exceeds 0.0080%, the workability deteriorates, so 0.0080% is made the upper limit.

If Si is reduced to less than 0.002%, the manufacturing cost is significantly increased and the economical efficiency is impaired.
The lower limit is 0.002%, and if it exceeds 1.0%, the workability deteriorates, so 1.0% is made the upper limit.

[0009] If Mn is reduced to less than 0.02%, the manufacturing cost is dramatically increased and the economic efficiency is impaired.
The lower limit is 0.02%, and if it exceeds 3.0%, the workability deteriorates, so the upper limit is 3.0%.

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 will increase dramatically and the economic efficiency will be impaired.
The lower limit is 0.002%, 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 plate, cracking during pressing, and surface defects. The lower limit is 0.002%, and 0.100%
If it exceeds, the workability will deteriorate. 0.100%
Is the upper limit.

N is preferably as small as possible, but 0.0
If it is less than 002%, the manufacturing cost will increase, so 0.0002% is the lower limit, and if it exceeds 0.0100%, age hardening becomes high and workability deteriorates.
The upper limit is 0.0100%.

When the content of Ti is less than 0.003%, the amount of carbonitrides formed is insufficient and the effect of improving workability is lost, so 0.003% is the lower limit, and
If it exceeds 0.100%, the workability deteriorates, so 0.100% is made the upper limit.

If the amount of Nb is less than 0.003%, the amount of carbonitrides formed is insufficient and the effect of improving workability is lost. Therefore, the lower limit is 0.003%, and
If it exceeds 0.100%, the workability deteriorates, so 0.100% is made the upper limit.

The surface layer component contains B in addition to the inner layer component. If the amount of B is less than 0.0003%, the effect of improving the brazing crack resistance is lost, so 0.0003 is used.
% Is the lower limit, and when it exceeds 0.0050%, the workability deteriorates, the recrystallization temperature rises, and annealing at a high temperature is required during annealing, which makes manufacturing difficult, so the upper limit is 0%. .0050%.

In the present invention, the above surface layer component is used,
The surface layer composition is adjusted in the continuous casting stage. That method, the molten 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 in the width direction in the mold above the molten steel injection position, The rising flow of the molten steel is decelerated, and a wire containing the iron alloy of B is added to the molten steel above the position where the magnetic field is applied
In this method, the molten steel in the upper part is made to have the above-mentioned surface layer composition by stirring the injected gas, and the slab having the above-mentioned composition is formed 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 in a state of being connected to a tundish (not shown). 3, a static magnetic field (N pole) 5 and a static magnetic field 5a for applying a static magnetic field in the mold 3 above the injection port 6 at the lower end of the immersion nozzle 4 at the molten steel injection position.
The (S pole) 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.
A static magnetic field is applied to the molten steel injected into the inside of the mold 3 by adding the element 9 (B) to be added to the molten steel 11 to be the surface layer while decelerating the upward flow of the molten steel by this static magnetic field. The molten steel in the upper part of 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. Then, when the molten steel 11 injected into the mold 3 from the immersion nozzle 4 is injected vertically (downward) from the injection port 6 of the immersion nozzle 4 together with the gas, the molten steel 11 rises up and down as shown by the arrow in the mold 3. The flow 12 moves upward, and the static magnetic fields 5 and 5 a above the injection port 6 provide the static magnetic field. When the static magnetic field is applied in this way, 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 becomes fine bubbles 13 that are finely dispersed and rise in the entire area of the molten steel. Above the added injection port 6, the gas is added. 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 out downward as, and no additional element is added when the alloy 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 downward direction, 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 ascending flow can be 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 in the lower part of the mold 3 has a static magnetic field blocking action and the rising of the molten steel itself. 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 single-hole type as shown in FIG. 1 or a multi-hole type (two or more holes). 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 0.015 to 0.15 t, which is the total thickness t per surface. The reason for this is that if the surface layer thickness is less than 0.015 t per one side, the thickness is not sufficient to secure brazing cracking resistance.
If the lower limit is 0.015 t and if it exceeds 0.15 t, the proportion of the layer containing a high alloy component increases, and the workability of the entire steel sheet deteriorates. Therefore, the upper limit is 0.15 t.

Specifically, when a static magnetic field is installed 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 10
The contact time of the surface with the mold 3 becomes longer. Therefore, the cooling time becomes longer, and the thickness of the surface layer 10a, which becomes the solidified layer, becomes thicker. Conversely, the higher the speed, the shorter the contact time of the surface of the molten alloy steel 10 in the mold 3 and the cooling. This is because the time to be shortened 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. The heating is from 900 ℃ to 130
It is desirable to set the temperature to about 0 ° C. Depending on the application of the steel sheet,
When the temperature of the slab is 1000 ° C. or higher, heating may be omitted. The hot rolling (including the case of not heating) performed after heating may be hot-rolled below the A ₃ transformation point, but it is preferably at least the A ₃ transformation point because sufficient workability cannot be obtained. After hot rolling, the hot rolled steel sheet is wound at an appropriate temperature. The product can be used as it is.

If a steel sheet having a thinner sheet thickness or higher workability is required, descaling treatment such as pickling is performed, cold rolling of 50% or more is performed, and then recrystallization temperature. Annealing is performed at the above temperature to form a cold rolled steel sheet.
After annealing, skin pass rolling is performed to obtain a product.

The multi-layer steel sheet of the present invention is a hot-rolled steel sheet, a cold-rolled steel sheet, and one or both surfaces of which are hot-dipped and / or electroplated for the purpose of improving corrosion resistance and weldability. Are possible and do not depart from the invention. Further, it is of course possible to subject the multilayer steel sheet of the present invention to various treatments, for example, chromate treatment, phosphate treatment, treatment for improving phosphate treatment, lubricity improving treatment, and weldability improving treatment. Even if a resin film treatment or the like is performed, it does not depart from the scope of the present invention, and various treatments can be additionally performed depending on the required characteristics.

[0029]

【Example】

[Example 1] Molten steel having the components listed in Table 1 was prepared as an inner layer component.

[Table 1]

Then, it was cast by the following method to obtain a steel plate. 1) Mold size 245 mm (short side) x 1200 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, while the wire B was added from the static magnetic field into the upper molten steel. At the same time, 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 rate of the B wire, the Cu content of the surface layer portion is 0.05 to 3.56%, and the thickness of the surface layer portion is 3.8.
A multi-layer slab having a uniformly formed surface layer portion of ˜33 mm was obtained.

Next, the slab was heated to 1050 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 2.9 to 4.8 mm. After removing the scale by pickling the steel sheet, the inner layer components 1 to 15
Was subjected to cold rolling to obtain a steel plate having a thickness of 1.0 to 1.2 mm. Then, perform continuous annealing at 800 ° C for 60 seconds,
0.8% skin pass rolling was performed to obtain cold-rolled steel sheets with symbols 1 to 7. In addition, 8-12 of Table 2 shows the evaluation result of a hot-rolled steel plate.

[0033]

[Table 2]

As a comparative example, a steel sheet having no B added to its surface was manufactured. The brazing crack resistance and workability of these steel sheets were evaluated. Brazing crack resistance is Cu / 6
Using brass braze containing 0%, Zn / 40%, 10
Brazing was performed at a temperature of 50 ° C., and the cross section after brazing was observed with a microscope to examine whether or not Cu—Zn entered the grain boundaries in the surface layer. If Cu-Zn entered the grain boundaries, it was determined that the resistance to brazing cracking was poor.

The workability was evaluated by using a circular steel plate punched 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 2 shows the evaluation results
Shown in From this table, it can be seen that the steels of the present invention are superior in brazing crack resistance, even though the limiting drawing ratio shows a high value of 1.8 or more as compared with the comparative examples.

[0036]

According to the present invention, the workability is not impaired,
It is possible to obtain a steel sheet having excellent brazing crack resistance. A large amount of B, which is an element that improves brazing cracking resistance, is contained in the surface layer, and a large amount of alloying elements is not contained inside the steel sheet. You can Further, in the method for manufacturing the steel sheet, the control of the surface layer thickness can be accurately performed by the withdrawing speed of the slab and the static magnetic field installation position, so that a steel sheet having a stable surface layer thickness and excellent in brazing crack resistance is provided. It is possible.

[Brief description of drawings]

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

FIG. 2 is a plan view illustrating a 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 number Office reference number FI technical display location B32B 15/01 B32B 15/01 A C22C 38/06 C22C 38/06 38/14 38/14

Claims (2)

[Claims] 1. In a multi-layer steel sheet having different components in the surface layer and the inner layer, C: 0.0002 to 0.0080%, Si: 0.002 to 1.00%, Mn: 0 in% by weight as the inner layer component. 0.02 to 3.0%, P: 0.002 to 0.150%, S: 0.002 to 0.050%, Al: 0.002 to 0.100%, N: 0.0002 to 0.0100. %, One or two of Ti: 0.003 to 0.100 and Nb: 0.003 to 0.100% of the carbonitride forming element group, and the balance: Fe and unavoidable impurities. If included and the plate thickness is t, 0.015t to 0.15t per surface from the front and back surfaces
As the surface layer component of the portion B, in addition to the inner layer component, B:
A multi-layer steel sheet containing 0.0003 to 0.0050% and having excellent brazing crack resistance. 2. 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 continuous casting mold, and a static magnetic field is applied to the entire widthwise direction in the mold above this molten steel injection position. To slow down the rising flow of the molten steel,
B is added to the molten steel above the position where the magnetic field is applied, the molten steel in the upper part is made to become the surface layer component according to claim 1 by stirring the injected gas, and the component according to claim 1 is extracted by mold drawing. No slab with composition, then,
Based on a conventional method for producing a steel sheet, hot rolling is performed, or hot rolling, descaling, cold rolling, and annealing are performed, and the inner layer has the components according to claim 1, and the sheet thickness is t. From 0.01
A method for producing a multi-layer steel sheet excellent in brazing cracking resistance, characterized in that a surface layer component of 5t to 0.15t is a hot rolled steel sheet or a cold rolled steel sheet containing the surface layer component according to claim 1.