JPH09291335A - Duplex-layer steel plate, excellent in surface characteristic and fatigue characteristic, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve surface characteristic and fatigue characteristic by incorporating specific amounts of C, Si, Mn, P, S, Al, N, and Fe into the inner layer part and additionally incorporating specific amounts of Ni into a part of specific thickness in the surface layer. SOLUTION: The inner layer has a composition consisting of, by weight, 0.02-0.15% C, 0.2-3% Si, 0.6-3% Mn, 0.002-0.03% P, 0.001-0.05% S, 0.002-0.1% Al, 0.0002-0.01% N, and the balance essentially Fe. The surface layer has a composition constituted by additionally incorporating 0.06-2% Ni into the composition of the inner layer. At this time, the surface layer means respective parts in the regions from the surface and the rear side surface to positions at a depth of 0.015-0.15t per side from the surface and the rear side surface, respectively, when plate thickness is represented by (t). A molten steel 11 is poured into a mold 3 via an immersion nozzle 4, together with gas downward perpendicularly. The molten steel 11 is reversed by the buoyancy of the gas and allowed to ascend as an ascending stream 12. Since a magnetostatic field 5 suppresses the ascending stream 12, and an alloy steel, in the part higher than the magnetostatic field 5, is allowed to flow stably, a surface layer 10a into which an Ni wire 9 is penetrated is formed stably on the surface of an inner layer 11a.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-layer steel sheet having excellent surface properties and fatigue properties and a method for producing the same, and particularly to a hot-rolled steel sheet used for automobile wheels, suspension parts and the like, and its production. It is a suitable method.

[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 a steel, a large amount of Si is added 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]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention
It is intended to eliminate the inferior surface property of a steel sheet containing a large amount of aluminum and to further improve the fatigue characteristics.

[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 3.00
%, P: 0.002-0.030%, S: 0.00
1 to 0.050%, Al: 0.002 to 0.100%,
N: 0.0002 to 0.0100% Remainder: Fe and unavoidable impurities are contained, and when the plate thickness is t, 0.015t to 0.15t per surface from the front and back surfaces.
In addition to the inner layer component,
i: a multi-layer steel sheet containing 0.060 to 2.00% and having excellent surface properties and fatigue properties, and a multi-layer steel sheet whose inner layer structure mainly comprises two phases of ferrite and martensite, and As a manufacturing method, molten steel having the above-mentioned composition is injected vertically downward or obliquely downward together with gas into a mold for continuous casting, and a static magnetic field is applied to the entire widthwise width in the mold above this molten steel injection position. Slows down the ascending flow of Ni to the molten steel above the position where the magnetic field is applied.
Is added so that the molten steel in the upper part becomes the above surface layer component by stirring the injected gas, and by drawing the mold, a slab having the above component composition is formed, and then hot rolling is performed to obtain a hot rolled steel plate. In the method for producing a multi-layer steel sheet having excellent surface properties and fatigue properties, and in hot rolling, hot rolling is finished at a finishing temperature of Ar ₁ transformation point or higher and air cooling is performed or immediately after completion (Ar ₁ transformation point From the temperature range above -30 ℃,
200 ° C at a cooling rate of 15 ° C / s or more and 200 ° C / s or less
A method for producing a multi-layered steel sheet having excellent surface properties and fatigue characteristics, characterized in that the hot-rolled steel sheet is cooled to the following temperature and has the above-mentioned composition, and the structure of the inner layer is mainly composed of two phases of ferrite and martensite It is in.

Si is an element that improves the ductility of 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.

The present invention has been made to solve such a drawback. As a result of various studies on the influence of the third element on the formation of Si scale, the inventors of the present application have found that the inclusion of Ni forms a uniform scale layer at the interface between the so-called Si scale and the base metal, resulting in descaling. It was also found that non-uniform scale removal does not occur during pickling and the generation of spot patterns is suppressed. The invention of the present application is based on this knowledge, and the basic technical idea is that Ni is contained in a large amount only in the surface portion of the steel sheet which is particularly effective for improving the surface texture, and unevenness caused by the formation of Si scale is caused. It is to suppress the pattern and improve the surface texture and eventually the fatigue property. By forming a uniform layer at the interface between the Si scale and the base iron, the surface becomes smooth because no trace of the Si scale remains after pickling.
Improves fatigue strength. 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.

The reasons for limiting the components will be described below. C
Is less than 0.02%, even if a large amount of elements such as Mn and Si that promote transformation structure strengthening are contained, the fraction of the martensite structure that contributes to strength improvement does not increase and sufficient strength cannot be achieved. Therefore, the lower limit is 0.02%. Also,
If it exceeds 0.15%, the fraction of martensite becomes too high, resulting in poor workability in processing into parts, so 0.15% is made the upper limit. If Si is less than 0.20%, ductility is lowered and the necessary amount of martensite cannot be secured, so 0.20% is the lower limit.
Further, if the content exceeds 3.00%, the workability is rather deteriorated, so the upper limit is 3.00%.

If Mn is less than 0.60%, ductility is deteriorated and a necessary amount of martensite cannot be secured, so the lower limit is 0.60%. 3.00%
If it is contained in excess of 0.1%, the workability will rather deteriorate, so the upper limit is 3.00%. P is 0.00
If it is reduced to less than 2%, the manufacturing cost will be drastically increased and the economic efficiency will be impaired.
If it is 030% or more, the weldability deteriorates, so 0.030% is made the upper limit.

If S is reduced to less than 0.001%, the manufacturing cost will be dramatically increased and the economy will be impaired.
The lower limit is 0.001%, and if it exceeds 0.050%, the workability deteriorates, so 0.050% is the upper limit. Al is
If it is less than 0.002%, deoxidation is insufficient and blowholes are generated in the steel, deteriorating the cleanliness of the steel sheet.
0.002% as it causes cracks and surface defects during pressing
Is set as the lower limit, and when it exceeds 0.100%, workability deteriorates, so 0.100% is set as the upper limit.

It is preferable that N is as small as possible, but 0.0
If it is less than 002%, the manufacturing cost is increased, 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%. Ni added to the surface layer is
If it is less than 0.060%, a uniform scale layer is not formed at the interface between the Si scale and the base iron, so 0.060% is made the lower limit. Further, if the content exceeds 2.00%, the workability deteriorates, so 2.00% is made the upper limit. 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 added,
The surface layer components are 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 slowed down, a wire containing an iron alloy of Ni is added to the molten steel above the position where the magnetic field is applied, and the molten steel in the upper portion is made to become the above-mentioned surface layer component by stirring the injected gas. , A method of forming a slab having the above-mentioned composition by drawing the mold.

This method will be described with reference to the drawings. FIG.
In FIG. 2, a continuous bottom casting mold 3 consisting of a long-side mold 1 and a short-side mold 2 is provided with a lower end open type immersion nozzle 4 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, 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 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 9 (Ni) to be added to the molten steel 11 to be the surface layer is added by decelerating the rising flow of the molten steel by this static magnetic field, The molten steel is 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.

When the molten steel 11 injected from the immersion nozzle 4 into the mold 3 is injected vertically together with the gas from the injection port 6 of the immersion nozzle 4 in the vertical (downward) direction, the molten steel 11 shows an arrow in the mold 3. 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 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 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 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. 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.

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 upward magnetic field is rapidly suppressed by applying the static magnetic field by the static magnetic fields 5 and 5a, so that the alloy steel 10 above the static magnetic fields 5 and 5a is greatly disturbed. There is no mold 3
The molten steel 11 in the lower part of the inside is prevented from being mixed with the molten steel alloy 10 by the static magnetic field blocking action and the rising flow 12 of the molten steel self, and the alloy steel 10a is reliably and stably formed on the surface of the inner layer 11a of the steel. It is possible to obtain the multi-layer cast slab 7. However, the injection port of the immersion nozzle may be a single-hole type as shown in FIG. 1 or a multiple-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 0.005 to 0.10 t, which is the total thickness t per surface. The reason for this is that if the surface layer thickness is less than 0.005 t per side, the amount of Ni required to suppress Si scale generation in the heating stage before hot rolling is not sufficiently secured, so 0.005 t is the lower limit. . On the other hand, if it exceeds 0.15 t, the proportion of the layer containing a high alloy component increases, and the strength of the entire steel sheet increases, resulting in impairing the workability. Therefore, 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 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, if the speed is low, the contact time of the surface of the molten alloy steel 10 with the mold 3 is correspondingly long, and accordingly, the cooling time becomes long, and the thickness of the surface layer 10a serving as a solidification layer becomes thicker. The higher the speed, the shorter the contact time of the surface of the molten alloy steel 10 in the mold 3 and the shorter the cooling time, and the surface layer 1 that becomes the solidified layer.
This is because the thickness of 0a becomes thin. 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 125
It is desirable to set it 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. 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. Usually, the hot rolled steel sheet is formed as it is, but in order to obtain higher fatigue strength, it is cooled by the following method after finishing hot rolling.
When the finishing temperature is high, after finishing hot rolling, air cooling is performed for 1 second to 50 seconds to promote the concentration 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 (Ar ₁ transformation point −30 ° C.)
From the above temperature range. 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 1
5 ° C./s or more and 200 ° C./s. If it is less than 15 ° C / s, the formation of martensite becomes insufficient, and if it is 200 ° C / s.
This is because if s is exceeded, the cooling equipment will become huge and the manufacturing cost will rise significantly. The ending temperature of the cooling is 200 ° C. or less. This is because if the temperature exceeds 200 ° C, the formation of martensite becomes insufficient. After that, descaling is performed by pickling or the like to obtain a product.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION 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. (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 inlet diameter φ90mm

[0022]

[Table 1]

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 iron alloy of Ni was contained in the molten steel above from the static magnetic field. Was added and a static magnetic field of 5000 gauss was applied, and casting was performed at a drawing speed of 1.3 m / min. By controlling the addition rate of the wire, a multilayer cast slab having a uniformly formed surface layer portion having a surface layer portion Ni content of 0.5% and a surface layer portion thickness of 20 mm was obtained. The slab is then heated to 1250 ° C. and then 84
Hot-roll at 0 ℃, air cool for 8 seconds, then 760 ℃ to 200 ℃
It was cooled at a cooling rate of 25 ° C./sec to a temperature equal to or lower than 0 ° C., and wound to obtain a hot-rolled steel plate having a plate thickness of 3.2 mm. The steel sheet was pickled to remove scale.

By the above method, a steel sheet having a 0.26 mm layer containing a large amount of Ni in the surface layer and having a two-phase structure in which the metal structure of the inner layer was 15% of martensite and the balance 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.

[0025]

Example As the inner layer component, molten steel having the components listed in Table 2 was prepared. 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 inlet diameter φ90mm

[0026]

[Table 2]

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 static iron magnetic field contained Ni iron alloy in the upper molten steel. And a drawing speed of 1.3 to 2 while applying a static magnetic field of 5000 Gauss.
It was cast at 0 m / min. By controlling the addition rate of the wire, the Ni content of the surface layer portion is 0.06 to 1.5.
A multilayer cast product having a uniformly generated surface layer portion having a surface layer portion thickness of 6% and a thickness of the surface layer portion of 3.9 to 31 mm was obtained. Then, the slab 1
Heat to 050 ~ 1250 ℃, then 840 ~ 910 ℃
Hot-rolling is performed, and after air cooling for 3 to 10 seconds, 20 to 100
It was cooled to 250 ° C or lower at a cooling rate of ° C / s and wound to obtain a hot-rolled steel sheet having a plate thickness of 2.9 to 5.6 mm. The steel sheet was pickled to remove scale.

As a comparative example, steel sheets (No. 12 to 16 in Table 3) in which Ni was not added to the surface layer were manufactured.
The fatigue properties of these steel sheets were evaluated. Fatigue properties were evaluated by determining the strength at the fatigue limit of 10 ⁷ times by the double swing plane bending fatigue test and using the ratio between this value and the tensile strength measured by static tension. 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.

[0029]

[Table 3]

[0030]

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 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 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)

Claims (4)

[Claims] 1. In a multi-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%, Mn: 0 by weight% as the inner layer component. .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: containing Fe and unavoidable impurities, and assuming that the plate thickness is t, as a surface layer component of 0.015 t to 0.15 t per surface from the front and back surfaces, in addition to the inner layer components, Ni: 0.060 A multi-layered steel sheet containing up to 2.00% and having excellent surface properties and fatigue characteristics. 2. In a multi-layer 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%, Mn: 0 in% by weight as the inner layer component. .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: containing Fe and unavoidable impurities, and assuming that the plate thickness is t, as a surface layer component of 0.015 t to 0.15 t per surface from the front and back surfaces, in addition to the inner layer components, Ni: 0.060 % To 2.00%, and the inner layer metallographic structure is mainly composed of two phases, ferrite and martensite, and has excellent surface properties and fatigue properties. 3. A molten steel having the inner layer component according to claim 1 is injected vertically downward or obliquely downward together with a gas into a mold for continuous casting, and a static magnetic field is applied to the entire width direction in the mold above the position where the molten steel is injected. To slow down the rising flow of the molten steel,
Ni 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 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. The slab has a structure. Then
A hot-rolled steel sheet having a surface layer component of 0.015t to 0.15t from the front and back surfaces, which is hot-rolled and has the components according to claim 1 in the inner layer and the plate thickness is t. A method for producing a multi-layer steel sheet having excellent surface properties and fatigue characteristics, characterized by: 4. 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 width direction in the mold above the position where the molten steel is injected. To slow down the rising flow of the molten steel,
Ni 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 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. The slab has a structure. Then
Hot rolling is performed at a finishing temperature of Ar ₁ transformation point or higher, and air-cooling is performed or immediately after completion (Ar ₁ transformation point −30 ° C.) or higher temperature range at a cooling rate of 15 ° C./sec or more and 200 ° C./s or less. When cooled to 200 ° C. or lower, the inner layer contains the component according to claim 1, and the plate thickness is t, 0.015 t from the front and back surfaces.
The surface properties and fatigue properties are characterized in that the surface layer component of 0.15 t contains the surface layer component according to claim 1 and the inner layer metallographic structure is a hot rolled steel sheet mainly composed of two phases of ferrite and martensite. An excellent method for manufacturing multi-layer steel sheets.