JPH08283897A - Multi-layered steel sheet excellent in workability and fatigue characteristic and its production - Google Patents

Abstract

PURPOSE: To produce a multi-layered continuously cast slab for high strength steel sheet excellent in workability and fatigue characteristics by providing a surface layer part, containing metallic elements for improving fatigue characteristics, onto the outside of a continuously cast slab at the time of continuously casting a molten low carbon steel. CONSTITUTION: A molten low carbon steel 11, having a composition containing, by weight, 0.01-0.20% C, 0.02-1.00% Si, 0.02-3.0% Mn, 0.002-0.150% P, 0.002-0.050% S, etc., is cast in a mold 3 for continuous casting via an immersion nozzle 4, and a gas is blown through a gas-blowing whole 8 of the nozzle 4 to agitate the molten metal. Simultaneously, while decelerating the ascending flow of the molten steel 11 by the action of a magnetostatic field 5 provided to the position above a lower end inlet 6 of the immersion nozzle 4, one or more elements among Nb, Ti, V, and Mo, as elements for improving fatigue strength, are added in the form of a wire 9. The resultant multi-layered continuously cast slab 7, which has an inner later 11a composed of low carbon steel and a surface layer 10a containing fatigue strength improving elements, formed on the outside of the inner layer 11a to a thickness 0.015-0.15 times the thickness of the cast slab 7, is drawn out from the mold 3 and rolled. By this method, the multi-layered steel sheet having excellent characteristics can be produced.

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 fatigue characteristics of a steel sheet, and a steel sheet as disclosed in, for example, JP-A-63-282240 is known. However,
Such a steel has a drawback in that it is necessary to add many elements or to utilize transformation structure strengthening in order to increase the strength, and although the strength can be obtained, the workability is deteriorated. In addition, a general low carbon steel plate having good deep drawability, which is applied to a part requiring workability, has a drawback that its fatigue property is inferior to that of a high strength steel plate.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a multi-layer steel sheet and a method for producing the same, which solves the drawback that workability is deteriorated when the fatigue characteristics of conventional low carbon steel are improved. is there.

[0004]

The gist of the present invention is as follows. (1) In a multilayer steel sheet having different components in the surface layer and the inner layer, C: 0.01 to 0.2 by weight% as the inner layer component.
0%, Si: 0.002-1.00%, Mn: 0.02
.About.3.0%, P: 0.002 to 0.150%, S: 0.
002-0.050%, Al: 0.002-0.100
%, N: 0.0002 to 0.0100%, the balance: Fe and unavoidable impurities, and when the plate thickness is t, 0.015 to 0.15 t of surface layer component per surface from the front and back surfaces,
In addition to the inner layer components, Ti: 0.
01 to 0.50%, Nb: 0.01 to 0.50%, V:
A high-strength multi-layer steel sheet excellent in workability and fatigue properties, containing one or more of 0.10 to 1.00% and Mo: 0.20 to 2.00%.

(2) Molten 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 of the mold above the position where the molten steel is injected. The rising flow of the molten steel is decelerated, and Ti, Nb, V, M is supplied to the molten steel above the position where the magnetic field is applied.
One or two or more of o are added, the molten steel in the upper part is made to have the above-mentioned surface layer composition by stirring the injected gas, and a slab having the above-mentioned composition is formed by drawing the mold, and then steel plate production According to the usual method of hot rolling,
Alternatively, when hot rolling, descaling, cold rolling, and annealing are performed, and the inner layer has the above-mentioned inner layer component, and the plate thickness is t, 0.015 t to 0.15 t of the surface layer component from one side to the other side is the above-mentioned. A method for producing a high-strength multi-layer steel sheet having excellent workability and fatigue characteristics, which comprises forming a hot-rolled steel sheet or a cold-rolled steel sheet containing surface layer components.

Ti, Nb, V, Cr and Mo are elements which increase the strength and improve the fatigue strength. However, if these elements are contained in a large amount, workability deteriorates. The feature of the present invention is that Ti, Nb, V, Cr, M
O is limited to exist only in the surface area of the steel sheet that is particularly effective for fatigue strength, and the inner layer remains low carbon steel to ensure fatigue strength without impairing workability.

[0007]

The reason for limiting the components of the inner layer will be described below. C
Is less than 0.01%, the steel sheet becomes too soft,
Even if the surface layer is made to contain a large amount of elements to improve the fatigue strength, the fatigue strength as a component requiring general fatigue characteristics cannot be secured, so 0.01% is made the lower limit. Also,
If it exceeds 0.20%, the workability is significantly deteriorated.
The upper limit is 0.20%.

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 3.0% is made the upper limit.

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%
Therefore, the workability deteriorates, so 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%.

Next, Ti, Nb, V, Cr, Mo on the surface layer
The reasons for limitation are described below. If the content of Ti is less than 0.01%, the amount of carbonitrides formed is insufficient and the effect of increasing strength is lost.
If it exceeds 0.1%, the amount of precipitates increases, the workability is deteriorated, and the effect of increasing strength is saturated, so 0.50% is made the upper limit.

If the amount of Nb is less than 0.01%, the amount of carbonitrides formed is insufficient and the effect of increasing the strength is lost, so 0.01% is the lower limit, and if it exceeds 0.50%, the precipitates are formed. The upper limit is 0.50% because the amount increases and the workability deteriorates and the effect of increasing strength is saturated.

If V is less than 0.10%, the amount of carbonitrides formed is insufficient and the effect of increasing the strength is lost, so the lower limit is 0.10%, and if it exceeds 1.00%, the precipitates are formed. Since the amount becomes large, workability is deteriorated, and the effect of increasing strength is saturated, the upper limit is 1.00%.

Cr and Mo increase the strength of steel mainly by solid solution strengthening. If Cr is less than 0.05%, the effect of increasing strength is lost, so 0.05% is the lower limit, and
If it exceeds 00%, the workability deteriorates, so 5.00% is made the upper limit. If Mo is less than 0.10%, the effect of increasing strength is lost, so 0.10% is the lower limit, and if it exceeds 2.00%, workability deteriorates, so 2.00% is the upper limit.

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, 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 this molten steel injection position, Decelerate the upward flow of molten steel. A wire containing an iron alloy of Ti, Nb, V, Cr, Mo is added to the molten steel above the magnetic field application position individually or in combination of two or more,
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, a lower end open type immersion nozzle 4 is arranged in a continuous casting mold 3 including a long side mold 1 and a short side mold 2 in a state of being connected to a tundish (not shown). Further, on the outside of the casting mold 3, above the casting inlet 6 at the lower end of the immersion nozzle 4 which is the molten steel pouring position, the casting mold 3 is
Static magnetic field (N pole) 5 and static magnetic field 5a that give a static magnetic field inside
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 molten steel, and the static magnetic field slows down the upward flow of the molten steel while adding elements 9 (Ti, Nb,
(V, Cr, Mo) is added to form the upper molten steel in the mold 3 into 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 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. A stream 12 moves upward, and a static magnetic field is given by the static magnetic fields 5 and 5a above the inlet 6.

When a static magnetic field is applied in this way, the molten steel 11
The reversed upflow of No. 1 is rapidly decelerated, but the decelerated deceleration moves the molten steel 11 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 alloy molten steel 10 is added. Becomes 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 finely dispersed as bubbles 13 and rises in the entire region in the molten steel, and the added element is present above the added injection port 6. 9 is agitated 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 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 is also blocked by the static magnetic field 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, 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 side, sufficient strength increase for securing fatigue properties cannot be obtained,
If the lower limit is 0.015t and the upper limit is more than 0.15t, the proportion of the layer containing a high alloy component increases, and the strength of the steel sheet as a whole increases, deteriorating the workability as the object of the present invention. The upper limit.

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 10.
It can be controlled to 30 mm, and the lower the drawing speed, the thicker the surface layer, and the higher the speed, the thinner the surface layer. That is, the lower the speed, the longer the contact time of the surface of the molten alloy steel 10 with the mold 3, and
As the cooling time becomes longer, the thickness of the surface layer 10a which becomes the solidified layer becomes thicker, and 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 time. Surface layer 10a which becomes a solidified layer by shortening
This is because the thickness of is reduced.

Since the properties of the steel sheet of the present invention are not changed by mixing the components at the boundary between the inner layer and the surface layer,
Mixing of surface and inner layer components 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 be performed at 900 ° C. to 1300 ° C. at which hot rolling can be performed.
Depending on the use of the steel sheet, heating may be omitted when the temperature of the slab is 1000 ° C. or higher.

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 not less than A ₃ transformation point because sufficient workability cannot be obtained. To do. After hot rolling, the hot rolled steel sheet is wound at an appropriate temperature. The product can be used as it is, but if a steel plate with a thinner plate thickness or higher workability is required, then after performing descaling treatment such as pickling, cold rolling of 50% or more is performed. Then, it is annealed at a temperature higher than the recrystallization temperature to obtain 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.

The multi-layer steel sheet of the present invention can of course be subjected to various treatments, for example, chromate treatment, phosphate treatment, treatment for improving phosphate treatment, lubricity improving treatment, and welding. Even if the property improving treatment, the resin film treatment and the like 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.

[0032]

【Example】

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

[Table 1]

Then, a steel plate was formed by casting according to the following method. 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 pure magnetic steel wire or a part thereof was introduced into the molten steel from the static magnetic field. In the steel No. 1, Cu and Ni wires were added, and 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 Cu and Ni wire in pure Cu wire or some steels, the Cu content in the surface layer portion is 0.0
A multi-layer cast slab having a uniformly generated surface layer portion having a surface layer portion thickness of 5 to 3.56% and a surface layer portion thickness of 3.8 to 33 mm was obtained.

Then, 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 the steel sheet was pickled to remove the scale, the inner layer components 1 to 15 were cold-rolled to give a steel sheet 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 19. The symbols 20 to 50 are the evaluation results of the hot rolled steel sheet.

The fatigue characteristics and workability of these steel sheets were evaluated. Fatigue property is 10 by the double swing plane bending fatigue test.
The strength at the ^7th fatigue limit was determined and evaluated by the ratio between this value and the tensile strength measured by static tension. Workability is φ
Evaluation was performed by using a circular steel plate punched out to 80 mm, performing flat-bottom cylindrical deep drawing with various drawing ratios, and determining the limiting drawing ratio.

The evaluation results are shown in Table 2. From this table, it is understood that the steels of the present invention have high fatigue characteristics of the fatigue limit ratio of 0.50 or more, even though the limit drawing ratio shows a high value of 1.8 or more as compared with the comparative example. .

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

According to the present invention, a large amount of Cu, which is an element for improving fatigue strength, is contained in the surface layer, and a large amount of alloying elements is not contained in the steel sheet, so workability is less deteriorated and fatigue characteristics are excellent. It is possible to obtain a steel plate. Further, in the method for producing a steel sheet, the control of the surface layer thickness can be accurately performed by the withdrawal speed of the slab and the static magnetic field installation position, so that a steel sheet having a stable surface layer thickness and excellent workability and fatigue characteristics 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: 0.01 to 0.20%, 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. %, Balance: containing Fe and unavoidable impurities, and assuming the plate thickness to be t, 0.015t to 0.15t per surface from the front and back surfaces.
In addition to the inner layer components, Ti: 0.01 to 0.50%, Nb: 0.01 to 0.50%, and V: 0.10 to 1. 00%, Mo: 0.10 to 2.00%, and a multi-layer steel sheet having excellent workability and fatigue properties, containing one or more of them. 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,
Ti, Nb, etc. to the molten steel above the position where the magnetic field is applied
One or two or more of Cr and Mo are added, the molten steel in the upper part is made to have the surface layer component according to claim 1 by stirring the injected gas, and the component composition according to claim 1 is obtained by drawing the mold. A slab is formed, which is then hot-rolled, or hot-rolled, descaled, cold-rolled, and annealed according to the usual method for producing a steel sheet, and the inner layer contains the components according to claim 1, and the sheet thickness is t. Then, 0.015t to 0.15t from the front and back
The hot-rolled steel sheet or the cold-rolled steel sheet containing the surface layer component according to claim 1 is used to produce a multi-layer steel sheet having excellent workability and fatigue properties.