JPH10204583A - Multilayer steel plate excellent in surface characteristic and fatigue characteristic, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem of inferior surface characteristic of the conventional high Si steel late and to improve fatigue characteristic to a greater extent. SOLUTION: In the multilayer steel plate where the composition of a surface layer is different from that of an inner layer, the inner layer has a composition consisting of, by weight, 0.02-0.15% C, 0.20-3.00% Si, 0.60-3.00% Mn, 0.002-0.030% P, 0.001-0.050% S, 0.002-0.100% Al, 0.0002-0.0100% N, and the balance Fe with inevitable impurities. Further, as to the composition of a surface layer in the part between a position at a depth, per side, of 0.015-0.15t, where (t) means plate thickness, and the surface and the rear surface, respectively, Ni is incorporated by 0.060-2.00% other than the components of the composition of the inner layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-layered steel sheet having excellent surface properties and fatigue properties, and a method for producing the same. Particularly, a hot-rolled steel sheet suitable for automobile wheels and underbody parts, and a method for producing the same. It is related to.

[0002]

2. Description of the Related Art Numerous attempts have been made to improve the fatigue strength of steel sheets, and the advantages of strengthening the composite structure with respect to the fatigue strength have been recognized. For example, Japanese Patent Application Laid-Open Nos. 60-145355 and 60-43425 disclose the advantages. BACKGROUND ART A composite structure reinforced high-strength steel sheet as disclosed in a gazette is known. However, in such steels, a large amount of Si is required to improve ductility.
At the hot rolling stage.
It is difficult to remove by pickling, which is called a scale, and even if it falls, a strong scale is left with fine irregularities remaining on the surface. The Si scale is not uniformly generated, but is unevenly formed on the surface of the steel sheet. For this reason, the conventional composite structure reinforced high-strength steel sheet has a defect that the surface appearance is inferior and the fatigue strength is lower than the performance originally provided by the composite structure steel sheet due to the Si scale.

[0003]

SUMMARY OF THE INVENTION The present invention relates to a conventional S
The purpose of the present invention is to eliminate the inferior surface properties of a steel sheet containing a large amount of i and to further improve the fatigue properties.

[0004]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a multi-layer steel sheet having different components in the surface layer and the inner layer, wherein C: 0.02 to 0.15% by weight as an inner layer component,
i: 0.20 to 3.00%, Mn: 0.60 to 3.0
%, P: 0.002 to less than 0.030%, S: 0.
001 to 0.050%, Al: 0.002 to 0.100
%, N: 0.0002 to 0.0100%, balance: Fe
And unavoidable impurities, and assuming that the plate thickness is t, Ni: 0.06 in addition to the inner layer component as a surface layer component of 0.015 t to 0.15 t per surface from the front and back surfaces.
A multi-layered steel sheet having excellent surface properties and fatigue characteristics characterized by containing 0 to 2.00%, or a multi-layered steel sheet mainly composed of two phases of ferrite and martensite. .

Further, as a method of manufacturing the same, molten steel having the above-mentioned composition is injected vertically or obliquely downward together with a gas into a continuous casting mold, and a static magnetic field is applied to the entire width in the mold above the molten steel injection position. To reduce the ascending flow of the molten steel, Ni is added to the molten steel above the application position of the magnetic field, so that the molten steel in the upper portion becomes the above-mentioned surface layer component by stirring the injected gas, By squeezing the mold, a slab having the above-mentioned composition is obtained.
Hot rolled, when manufacturing a multi-layered steel sheet with excellent surface properties and fatigue properties as a hot rolled steel sheet, or when performing the hot rolling,
The hot rolling is completed at a finishing temperature not lower than the Ar ₁ transformation point, and is subjected to air cooling, or immediately after the completion, from a temperature range of (Ar ₁ transformation point −30 ° C.) or higher, from 15 ° C./sec to 200 ° C./sec or lower. Cooling at a cooling rate of 200 ° C. or less, the composite having the above-mentioned composition and having an inner layer structure of a hot-rolled steel sheet mainly composed of two phases of ferrite and martensite. A method for manufacturing a laminated steel sheet.

[0006] Si is an element that improves ductility in a transformed structure strengthened steel sheet. However, when a large amount of Si is contained, a strong scale called Si scale is generated. This scale has a composition of 2FeO.SiO ₂ and is formed in a shape of deeply and irregularly penetrating into the surface of the steel sheet. Further, the portions to be formed are not uniformly formed on the surface of the steel sheet, but are formed in spots. This scale is less likely to fall off by pickling than a normal scale,
Even if it falls, fine irregularities remain on the surface, so that a mottled pattern remains on the steel sheet surface. This causes a Si scale pattern to deteriorate the surface properties. Also, due to the presence of fine surface irregularities after pickling,
There is a disadvantage that the fatigue strength of this part is deteriorated.

[0007] The present invention has been made to solve such a drawback. The present inventors have conducted various studies on the effect of the third element on the formation of Si scale. As a result, by adding Ni, a uniform scale layer was formed at the interface between the so-called Si scale and the ground iron, In addition, it was found that non-uniform scale was not removed at the time of pickling and the formation of spots was suppressed.

The present invention has been made based on this finding. The basic technical idea is that Ni is contained in a large amount only in the surface portion of a steel sheet which is particularly effective for improving the surface properties.
An object of the present invention is to suppress a mottled pattern caused by the formation of scale and to improve surface properties and, consequently, fatigue characteristics. By generating a uniform layer at the interface between Si scale and ground iron,
Since no trace of Si scale remains after pickling, the surface becomes smooth and the fatigue strength is improved. This alone increases the fatigue strength, but further increases the fatigue strength by making the metal structure of the inner layer a two-phase structure of ferrite and martensite.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
First, the reasons for limiting the components will be described. C is 0.02%
If it is less than 0.1%, even if a large amount of an element that promotes the strengthening of the transformed structure, such as Mn or Si, is contained, the fraction of the martensite structure contributing to the strength improvement does not increase, and the strength cannot be sufficiently improved. 02% is the lower limit. In addition, 0.15%
If the ratio exceeds 0.15%, the fraction of martensite becomes too high, and the workability when processing into a part becomes inferior.
% As the upper limit.

If the content of Si is less than 0.20%, the ductility is reduced, and the required amount of martensite cannot be secured. Therefore, the lower limit is set to 0.20%. 3.00%
If the content exceeds 0.1%, the workability is rather deteriorated, so the upper limit is 3.00%.

If the Mn content is less than 0.60%, the ductility is reduced, and the required amount of martensite cannot be secured. Therefore, the lower limit of Mn content is 0.60%. 3.00%
If the content exceeds 0.1%, the workability is rather deteriorated, so the upper limit is 3.00%.

Since reducing P to less than 0.002% dramatically increases manufacturing costs and impairs economic efficiency,
The lower limit is 0.002%, and if it is 0.030% or more, the weldability deteriorates, so 0.030% is made the upper limit.

[0013] Since reducing S to less than 0.001% dramatically increases the production cost and impairs the economic efficiency,
The lower limit is 0.001%, and if it exceeds 0.050%, the workability deteriorates, so the upper limit is 0.050%.

If Al is less than 0.002%, deoxidation is insufficient and blowholes occur in the steel, impairing the cleanliness of the steel sheet, causing cracking and surface flaws during pressing. 0.002% as the lower limit, and 0.100%
Exceeds 0.10, the workability deteriorates.
0% is the upper limit.

N is preferably as small as possible.
When the content is less than 002%, the production cost is increased. Therefore, the lower limit is 0.0002%, and when the content is more than 0.0100%, the age hardenability increases, and the workability deteriorates.
0.0100% is made the upper limit.

If the amount of Ni added to the surface layer is less than 0.060%, a uniform scale layer is not formed at the interface between the Si scale and the iron base, so the lower limit is 0.060%. Also,
If the content exceeds 2.00%, the workability deteriorates, so the upper limit is 2.00%. As a result, the surface layer component contains a large amount of Ni in addition to the inner layer component.

In the present invention, the above-mentioned surface layer component is
The surface layer components are adjusted in the continuous casting stage. The method involves injecting molten steel having the above-mentioned inner layer component into a continuous casting mold together with gas in a vertically downward or obliquely downward direction, and applying a static magnetic field to the entire width in the mold above the molten steel injection position to apply the static magnetic field. The ascending flow of the molten steel is slowed down, a wire containing a Ni iron alloy is added to the molten steel above the position where the magnetic field is applied, and the upper molten steel becomes the above-described surface layer component by stirring the injected gas. This is a method of forming a slab having the above-mentioned composition by drawing out a mold.

This method will be described with reference to the drawings. 1 and 2, in a continuous casting mold 3 composed of a long side mold 1 and a short side mold 2, a lower end open type immersion nozzle 4 is arranged in a state of being connected to a tundish (not shown). Outside the mold 3, the mold 3 is located above the injection port 6 at the lower end of the immersion nozzle 4 where the molten steel is injected.
Magnetic field (N pole) 5 and static magnetic field 5a for applying a static magnetic field inside
The (S pole) is arranged so as to cover the entire width of the long side mold 1, that is, the entire width of the slab 7. At the time of casting, molten steel 11 having the above-described inner layer component is injected into mold 3 by immersion nozzle 4, and gas is blown from gas inlet 8 of immersion nozzle 4 at the same time.

On the other hand, above the injection port 6 at the lower end of the immersion nozzle 4 where the molten steel 11 is to be injected, the static magnetic fields 5, 5a arranged so as to extend over the entire width of the long side mold 1 into the mold 3. An element (Ni) to be added to the molten steel 11 to be a surface layer is added by a wire 9 while applying a static magnetic field to the injected molten steel, and the upward flow of the molten steel is decelerated by the static magnetic field. The upper molten steel is made of an alloy containing the above surface layer components. Then, this is continuously cast and drawn downward as a slab 7, and as shown in FIG. 3, the above-described surface layer component is added only to the surface layer 10a, and the multilayer slab 7 in which the inner layer 11a is the inner layer component is cast.

Thus, 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 gas, as indicated by an arrow in the mold 3. It moves upward as a reversal rising flow 12,
Here, a static magnetic field is applied by the static magnetic fields 5 and 5a above the injection port 6. When the static magnetic field is applied as described above, the inverted upward flow of the molten steel 11 is rapidly decelerated, but the speed is reduced and the molten steel 11 moves to the upper part of the static magnetic fields 5, 5a. The element 9 to be added is added to form the molten alloy steel 10.

On the other hand, the gas injected in the vertical direction from the inlet 6 of the immersion nozzle 4 together with the molten steel 11 becomes finely dispersed as bubbles 13 and rises in the entire area of the molten steel. The obtained element 9 is stirred to form a uniform molten alloy steel 10. Then, from the mold 3 to the slab 7
When the alloy molten steel 10 above the static magnetic field 5, 5a is cooled down and solidified by being drawn downward, no additional element is added when the alloy steel 10 is drawn and moved below the static magnetic field 5, 5a. The steel obtained by solidification of the molten steel 11 is used as the inner layer 11a, and only the surface becomes the multilayer slab 7 in which the surface layer 10a of the alloy steel in which the solidified layer of the alloy molten steel 10 gradually expands with the drawing movement.

As described above, by injecting the molten steel 11 from the injection port 6 of the immersion nozzle 4 with the gas vertically downward,
After the injection flow of the molten steel 11 reaches downward, it reverses due to the buoyancy of the gas and rises as an ascending flow 12, but the flow velocity at this time becomes gentle near the static magnetic fields 5 and 5 a with the increase, and further immerses. Above the injection port 6 of the nozzle 4, the upward flow is rapidly suppressed by the application of the static magnetic field by the static magnetic fields 5, 5a. Therefore, the alloy steel 10 above the static magnetic fields 5, 5a is greatly disturbed. There is no mold 3
The alloy steel 10a is reliably and stably formed on the surface of the inner layer 11a of the steel without the alloy molten steel 10 being mixed into the molten steel 11 in the lower part of the inside by the static magnetic field blocking action and the rising flow 12 of the molten steel itself. The obtained multilayer slab 7 can be obtained. However, the injection port of the immersion nozzle may be the one-hole type shown in FIG. 1 or a multi-hole type (two or more holes). The 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 set to 0.015 to 0.15 t of the total thickness t on one side. The reason is that if the surface layer thickness is less than 0.015 t per side, the Ni amount required to suppress the generation of Si scale at the stage of heating before hot rolling is not sufficiently secured, so the lower limit is 0.015 t. . On the other hand, if it exceeds 0.15 t, the ratio of the layer containing a high alloying component becomes high, and the strength of the entire steel sheet becomes high, thereby impairing the workability. Therefore, the upper limit is 0.15 t.

Specifically, when a static magnetic field is set in a mold, the surface layer thickness is 10 to 2.0 m / min.
The thickness can be controlled to 30 mm. 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 therefore,
As the cooling time becomes longer, the thickness of the surface layer 10a serving as a solidified layer becomes thicker, and conversely, the higher the speed, the shorter the contact time of the surface of the alloy molten steel 10 with the mold 3 and the longer the cooling time. Is shortened, and the surface layer 10a becomes a solidified layer.
Is thinner.

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 components of the surface layer and the inner layer is permitted.

The slab thus obtained is formed into a steel sheet according to a conventional method for manufacturing a steel sheet. First, after continuous casting, the slab is cooled directly or once to an appropriate temperature, and then heated in a heating furnace. Heating is performed at 900 ° C to 12 ° C where hot rolling is possible.
It is desirable that the temperature be about 50 ° C. Depending on the use of the steel sheet, when the temperature of the slab is 1000 ° C. or higher, heating may be omitted. Hot rolling (including non-heating) performed after heating is hot-rolled at a finishing temperature equal to or higher than the Ar ₁ transformation point. Normally, the hot rolled steel sheet is used as it is, but after finishing hot rolling, it is cooled by the following method in order to obtain higher fatigue strength. When the finishing temperature is high, after finishing hot rolling, 1 second to 50 seconds
Air cooling is performed for 2 seconds to promote the enrichment of C in austenite, or when the finishing temperature is close to the Ar ₁ transformation point, cooling can be started immediately. The cooling start temperature is from a temperature range of (Ar ₁ transformation point −30 ° C.) or higher. This is because, at a temperature lower than this temperature, the generation of ferrite increases and the generation of martensite becomes insufficient. Cooling rate is 15 ℃ / sec or more and 200 ℃
/ sec. If the temperature is lower than 15 ° C./sec, the formation of martensite will be insufficient, and if the temperature exceeds 200 ° C./sec, the cooling equipment will be enormous and the production cost will increase significantly. The ending temperature of the cooling is 200 ° C. or less. If the temperature exceeds 200 ° C., the formation of martensite becomes insufficient. Thereafter, descaling is performed by pickling or the like to obtain a product.

[0027]

【Example】

(Example 1) As an inner layer component, molten steel having the components listed in Table 1 was prepared.

[0028]

[Table 1]

Then, casting was performed by 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
(3) Immersion nozzle inlet position: 50 mm below the static magnetic field position (4) Immersion nozzle inlet diameter: φ90 mm

Into such a continuous casting apparatus, molten steel containing the components shown in Table 1 is injected from a submerged nozzle into a mold together with 3.0 liter / min of Ar gas, while a static magnetic field is injected into the upper molten steel. A wire containing a Ni iron alloy was 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 rate of addition of the wire, a multilayer slab having a uniformly formed surface layer having a Ni content of the surface layer of 0.5% and a thickness of the surface layer of 20 mm was obtained.

Next, the slab is heated to 1250 ° C., then hot-rolled at 840 ° C., air-cooled for 8 seconds, cooled from 760 ° C. to 200 ° C. or less at a cooling rate of 25 ° C./sec, and wound up. A hot-rolled steel sheet having a thickness of 3.2 mm was used. The steel sheet was pickled to remove scale.

By the above method, 0.26 mm of N
A steel sheet having a layer containing a large amount of i and having a two-phase structure in which the metal structure of the inner layer is composed of 15% martensite and the balance of ferrite was obtained. No spot-like Si scale was observed on the surface of the steel sheet. When a plane bending fatigue test was performed on the steel sheet, a value obtained by dividing the maximum stress that would not cause fracture even in a fatigue test of 10 million times by the tensile strength of the base material, that is, the fatigue limit ratio was 0.53, which was excellent. It was confirmed that the steel showed a high fatigue characteristic.

Example 2 Molten steel having the components listed in Table 2 was prepared as the inner layer components.

[0034]

[Table 2]

Then, casting was performed under the same conditions as in Example 1. Into such a continuous casting apparatus, molten steel containing the components shown in Table 2 was injected from an immersion nozzle to a flow rate of 3.0 liter / min.
The gas is injected into the mold together with the gas. On the other hand, a wire containing the iron alloy of Ni is added from the static magnetic field into the upper molten steel, and a drawing speed of 1.3 to 2.0 m / is applied while applying a static magnetic field of 5000 Gauss. Cast in minutes. By controlling the rate of addition of the wire, the Ni content in the surface layer is 0.06 to 1.56%, and the thickness of the surface layer is 3.9 to 3%.
A multilayer slab having a uniformly formed surface layer of 1 mm was obtained.

Next, the slab was heated to 1050 to 1250 ° C., and then hot-rolled at 840 to 910 ° C.
After air cooling for 0 seconds, a cooling rate of 20 to 100 ° C / sec.
Cool down to 50 ° C or less, roll up and plate thickness 2.9-5.6
mm hot-rolled steel sheet. The steel sheet was pickled to remove scale. Further, as comparative examples, steel sheets without Ni added to the surface layer (Nos. 12 to 16 in Table 3) were manufactured.

The fatigue properties of these steel sheets were evaluated. The fatigue properties were evaluated by a swing plane bending fatigue test, in which a fatigue limit of 107 times was determined, and the ratio between this value and the tensile strength measured by static tension was evaluated. Further, the surface appearance after pickling was observed. The surface properties were evaluated based on whether or not there were traces of mottled Si scale on the surface.

Table 3 shows the evaluation results. From this table, it can be seen that the steel of the present invention has no trace of Si scale, has a fatigue limit ratio of 0.5 or more, and exhibits excellent fatigue characteristics, as compared with the comparative example.

[0039]

[Table 3]

[0040]

According to the present invention, since the surface layer contains a large amount of Ni for detoxifying the Si scale, there is no generation of Si scale, and the inside of the steel sheet becomes a steel sheet having excellent fatigue characteristics and ductility. It is possible to obtain a steel sheet having excellent properties and fatigue properties. Further, in the method for producing the steel sheet, the surface layer thickness can be controlled accurately at the slab drawing speed and the static magnetic field installation position, so that a steel sheet having a stable surface layer and excellent in surface properties and fatigue properties is provided. can do.

[Brief description of the 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 multilayer slab 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 injection port 8: Gas blowing port 9: Wire containing additional element 10a: Surface layer (surface layer component 11: Molten steel 11a: Inner layer (solidified layer of molten steel)

Claims (4)

[Claims] 1. In a double-layered steel sheet having different components in the surface layer and the inner layer, C: 0.02 to 0.15%, Si: 0.20 to 3.00%, and Mn: 0 by weight as inner layer components. 0.60 to 3.00%, P: 0.002 to 0.030%, S: 0.001 to 0.050%, Al: 0.002 to 0.100%, N: 0.0002 to 0.0100 %, The balance: Fe and unavoidable impurities, and assuming that the plate thickness is t, Ni: 0.1% in addition to the inner layer component as a surface layer component of 0.015 t to 0.15 t per surface from the front and back surfaces. A multi-layer steel sheet having excellent surface properties and fatigue characteristics, characterized in that the steel sheet contains 060 to 2.00%. 2. In a double-layered steel sheet having different components in the surface layer and the inner layer, C: 0.02 to 0.15%, Si: 0.20 to 3.00%, and Mn: 0 by weight as inner layer components. 0.60 to 3.00%, P: 0.002 to 0.030%, S: 0.001 to 0.050%, Al: 0.002 to 0.100%, N: 0.0002 to 0.0100 %, Balance: Fe and unavoidable impurities, and assuming that the plate thickness is t, in addition to the inner layer component, Ni: 0. A multi-layered steel sheet containing 060 to 2.00% and having an inner layer metal structure mainly composed of two phases of ferrite and martensite, and having excellent surface properties and fatigue properties. 3. A molten steel having an inner layer component according to claim 1 is injected into a mold for continuous casting in a vertically downward or obliquely downward direction together with a gas, and a static magnetic field is applied to the entire width in the mold above the molten steel injection position. To reduce the upward flow of the molten steel, add Ni to the molten steel above the position where the magnetic field is applied, and agitate the injected gas to turn the upper molten steel into a surface layer component according to claim 1. As a result, a slab having the component constitution according to claim 1 is formed by mold drawing, and then, the slab is subjected to hot rolling, the inner layer has the component according to claim 1, and the sheet thickness is t. 0.015t from the back
A method for producing a multi-layered steel sheet having excellent surface properties and fatigue properties, characterized in that a hot-rolled steel sheet having a surface layer component of up to 0.15 t has the surface layer component according to claim 1. 4. A molten steel having an inner layer component according to claim 1 is injected into a mold for continuous casting in a vertically downward or obliquely downward direction together with a gas, and a static magnetic field is applied to the entire width in the mold above the molten steel injection position. To reduce the upward flow of the molten steel, add Ni to the molten steel above the position where the magnetic field is applied, and agitate the injected gas to turn the upper molten steel into a surface layer component according to claim 1. A slab having the component constitution according to claim 1 is formed by drawing the mold, and then hot-rolled at a finishing temperature not lower than the Ar ₁ transformation point and air-cooled or immediately after completion (at the Ar ₁ transformation point). -30
2) cooling at a cooling rate of 15 ° C./sec or more and 200 ° C./sec or less from a temperature range of not less than 200 ° C. to an inner layer.
It has the components described, and when the plate thickness is t, 0.
Surface properties and fatigue properties characterized in that the surface layer component of 015t to 0.15t contains the surface layer component of claim 1, and the metal structure of the inner layer is a hot-rolled steel sheet mainly composed of two phases of ferrite and martensite. Method for manufacturing multi-layer steel sheets with excellent performance.