JP5121225B2 - Manufacturing method of high strength plated steel sheet for spot welding with excellent magnetic shielding - Google Patents

Description

  The present invention relates to a plated steel sheet for a structural component incorporated in home appliances, electronic devices, and the like, and relates to a high-strength plated steel sheet that exhibits high strength and excellent geomagnetic shielding even after spot welding.

  Plated steel sheets having a thickness of about 0.8 to 2.0 mm are mainly used for structural parts incorporated in home appliances, electronic devices, and the like. And such a structural component is spot-welded through steel plates or an appropriate joint plate after forming a plated steel plate into a suitable shape. After that, it is attached to the main body, but since static tensile stress is applied to the spot welded portion at the time of the attachment, the spot welded portion also needs high fracture strength.

On the other hand, when a steel plate is used as a member of a household electrical appliance or an electronic device, it also needs a function as a magnetic shield material that prevents the invasion of geomagnetism into the interior of the household electrical appliance or electronic device. That is, it is necessary that the magnetic permeability (μ _0.35 ) in a low magnetic field ( _0.35 Oe) as high as the geomagnetism is high.
Thus, steel plates used for members of home appliances, electronic devices and the like require high magnetic permeability in addition to high strength of the base material portion and high fracture strength of the spot welded portion in order to reduce weight. . In particular, since the plastic deformation resistance of structural parts becomes a problem, the yield strength of steel sheets is regarded as important.
However, there is a contradictory relationship between the increase in strength required for steel sheets and the increase in magnetic permeability. In other words, any of the strengthening methods such as precipitation strengthening by adding microalloys such as Ti and Nb and strengthening crystal grain refinement by refining the ferrite grain size, which are effective means for increasing the strength of the steel sheet, decrease the magnetic permeability. Let

Therefore, as seen in Patent Documents 1 and 2, in order to obtain a steel sheet that achieves both high strength and high magnetic permeability, which are contradictory properties, 1 mass% or more of Si effective for solid solution strengthening is added. And it is going to use the cold-rolled steel plate which made C 0.005 mass% or less, what is called a silicon steel plate.
Further, in Patent Document 3, the Ti and S contents of an ultra-low carbon steel having a C content of 0.005% by mass or less, which is solid solution strengthened with Si and Mn regardless of precipitation strengthening, are adjusted. Thus, it has been proposed that C, S, and N are fixed as Ti ₄ C ₂ S ₂ , TiN to prevent the domain wall from moving and to refine the ferrite grain size, thereby improving the magnetic permeability.

JP-A-10-208670 Japanese Patent Laid-Open No. 10-214578 JP-A-11-140601

However, the material characteristic necessary for improving the geomagnetic shielding property is magnetic permeability in a low DC magnetic field, and low iron loss in an AC magnetic field, which is a feature of a silicon steel sheet, is not necessary. In addition, the addition of a large amount of Si with an extremely low carbon content of 0.005 mass% or less increases the cost of steelmaking. Further, the addition of a large amount of Si significantly reduces the toughness and ductility of the steel sheet, so that cracking is likely to occur in hot rolling and cold rolling, and productivity is deteriorated. Furthermore, since a large amount of Si increases the A ₃ transformation temperature of the steel sheet, transformation from ferrite to austenite is suppressed in the vicinity of the spot weld. As a result, the ferrite is heated to a high temperature, and the ferrite grains are remarkably coarsened so that the spot welding strength is lowered. Therefore, the so-called silicon steel sheet utilization technique proposed in Patent Documents 1 and 2 is not realistic.

In the steel sheet described in Patent Document 3, since Ti is added, the recrystallization temperature is high, and the manufacturing cost is increased.
The present invention has been devised to solve such problems, has the same level of productivity as a general steel sheet, and requires the addition of a large amount of Si or the addition of precipitation strengthening elements such as Ti. An object of the present invention is to provide a high-strength plated steel sheet that achieves both high permeability and high strength of the base metal part and spot welded part.

The high-strength plated steel sheet for spot welding having excellent magnetic shielding properties according to the present invention is achieved by the following: C: 0.003-0.010 mass%, Si: 0.5-1.0 mass%, Mn : 1.0 to 2.0 mass%, P: 0.04 to 0.15 mass%, Al: 0.04 mass% or less, N: 0.004 mass% or less, the balance being Fe and inevitable impurities And the content of Si, Mn and P has a chemical composition satisfying the following formula (1), the ferrite grain size after passing through hot rolling, cold rolling, annealing and plating steps is 10 to 80 μm, and The magnetic permeability in a DC magnetic field of 0.35 Oe is 400 G / Oe or more.

(Si%) × 45− (Mn%) × 40 + (P%) × 150 ≦ 10
(1)
Moreover, such a high-strength plated steel sheet for spot welding having excellent magnetic shielding properties is rolled up at a temperature of 600 to 700 ° C. when hot rolling a steel slab having the above chemical composition, and after 750 to 750 It can be obtained by continuous annealing at a temperature of 800 ° C. and subsequent plating.

In the present invention, the A ₃ transformation point is lowered by adjusting the mutual relationship between the contained Si, Mn, and P amount while increasing the strength by solute C, Si, Mn, P, during spot welding. The increase in temperature in the vicinity of the welded portion in the steel can suppress the coarsening of the ferrite grain size and ensure the strength of the spot welded portion.
Therefore, according to the present invention, there is provided a plated steel sheet that does not have an increase in steelmaking cost as seen in a steel type in which a large amount of Si is added to a conventional ultra-low carbon steel sheet, and has excellent productivity and magnetic shielding properties. In addition, the required strength of the base material is given, and the strength reduction of the welded part is suppressed even when spot welding is performed, so high strength with excellent reliability for structural parts incorporated in home appliances and electronic devices, etc. A plated steel sheet is provided.

The inventors of the present invention have repeatedly studied means for improving the magnetic permeability μ _0.35 in a dc magnetic field as weak as geomagnetism and increasing the strength.
As a result, first, it was found that it is effective to reduce the precipitates in the steel as much as possible in order to improve the permeability in a low magnetic field. Specifically, there is no precipitation strengthening element such as Ti and Nb. It is necessary to reduce the amount of C and N after addition. In addition to the regulation of these steel components, it is important to control the ferrite grain size by limiting the production conditions. As the ferrite grain size becomes finer, the permeability μ _0.35 decreases, and conversely the base material strength increases. That is, in order to achieve high strength and high magnetic permeability at the same time, it has been found that it is important to strictly control the ferrite grain size together with the steel components. In order to obtain an appropriate ferrite grain size, appropriate control of various conditions in the hot rolling process and the annealing process is necessary.

On the other hand, in order to ensure the strength of the spot welded portion, it is necessary to suppress the coarsening of the ferrite grain size in the weld heat affected zone (HAZ portion). The ferrite grain size of the HAZ part is affected by spot welding conditions, but generally suitable welding conditions, that is, the nugget diameter is 4 × t ^1/2 or more with respect to the steel sheet thickness t, and The ferrite grain size in the HAZ part can be regarded as substantially constant as long as it is within the range of applied pressure, welding current, and energization time at which no dust is generated. That is, the prevention of the coarsening of the ferrite grain size in the HAZ part needs to be controlled not by the welding conditions but by the material to be welded. However, the addition of microalloys such as Ti and Nb is effective for preventing coarsening of the ferrite grain size in the HAZ part, but is not suitable for securing magnetic properties.
As a result of various studies, it was found that it is effective to lower the A ₃ transformation point. Specifically, by using the A ₃ transformation point lowering action of Mn and P added as a solid solution strengthening element to ensure strength, a certain limit is imposed on the amount of Si having A ₃ transformation point raising action. Was found to be effective.
Details will be described below.

First, the component composition of this invention steel plate is demonstrated.
C: 0.003 to 0.010 mass%
C is an alloy component effective for improving the strength. In the present invention, it is an essential component for increasing the strength of a steel sheet used for general home appliances, electronic devices and the like. A part of C becomes a solid solution C in a state of continuous annealing, and gives a high yield stress to the steel sheet. For this purpose, it is necessary to contain 0.003 mass% or more of C. Moreover, reducing the C content to less than 0.003 mass% increases the steelmaking cost. In order to use it for home appliances, electronic devices, etc., it is necessary to ensure magnetic properties. If a large amount of C exceeding 0.010 mass% is contained, the permeability μ _0.35 after receiving heat deteriorates.

Si: 0.5 to 1.0% by mass
Si is an effective component for increasing the strength of a steel sheet by solid solution strengthening. However, the desired high strength cannot be achieved unless the content is less than 0.5% by mass. However, when a large amount of Si is contained so as to exceed 1.0% by mass, not only the A ₃ transformation point is raised and the ferrite particle size of the weld heat affected zone (HAZ) is coarsened, but also the plating adhesion. Reduce.
Mn: 1.0 to 2.0% by mass
Mn is an effective component for increasing the strength of a steel sheet by solid solution strengthening. It is more effective than Si from the viewpoint of securing plating adhesion and lowering the A ₃ transformation point. However, the desired high strength cannot be achieved unless the content is less than 1.0% by mass. However, when Mn exceeding 2.0% by mass is contained, not only the workability is deteriorated but also the plating adhesion is lowered.

P: 0.04 to 0.15% by mass
P exhibits the effect of increasing the strength of the steel sheet by solid solution strengthening. However, the desired high strength cannot be achieved unless the content is less than 0.04 mass%. However, if it exceeds 0.15% by mass, segregation becomes prominent and the manufacturability deteriorates, and the toughness of the steel sheet significantly decreases.
Al: 0.04 mass% or less Al is effective as a deoxidizer for reducing inclusions in steel, but also has an effect of fixing N in steel as AlN to improve magnetic properties. In order to exert the above action, the content is preferably 0.005% by mass or more. However, when a large amount of Al is contained so as to exceed 0.04% by mass, the amount of AlN increases, and the grain growth of ferrite is suppressed to deteriorate the magnetic characteristics.

S: 0.02% by mass or less Steel is usually mixed with S as an inevitable impurity. This S is a harmful component for the steel sheet, and if it is present as an inclusion in the steel sheet, it adversely affects both the bending workability and the magnetic permeability of the steel sheet, so it is preferable to reduce it as much as possible. If the S content is reduced to 0.02 mass or less, the above adverse effects can be substantially avoided.
N: 0.004% by mass or less N is present in the steel sheet as a precipitate such as AlN, which inhibits the growth of ferrite grains and lowers the magnetic properties, so it is preferable to reduce it as much as possible. If the N content is reduced to 0.004% by mass or less, the above effect can be substantially avoided.

  Next, organizational features that provide desired mechanical and magnetic properties will be described.

Ferrite grain size: 10-80 μm
The refinement of the ferrite grain size leads to an increase in strength, and also leads to an effect of suppressing the refinement of the magnetic domain and the domain wall movement in the DC magnetic field. Specifically, in order to increase the magnetic permeability μ _0.35 to 400 G / Oe or more in order to improve the geomagnetic shielding property described later, the ferrite grain size needs to be 10 μm or more. On the other hand, the coarsening of the ferrite grain size reduces the yield stress. In order to ensure the plastic deformation resistance of the structural parts incorporated in home appliances, electronic devices, etc., it is preferable that the yield stress is 300 N / mm ² or more including the weld heat affected zone. The weld heat-affected zone tends to have large crystal grains, and in order to secure a yield stress of 300 N / mm ² or more, the ferrite grain size of the base material portion needs to be set to 80 μm or less in advance. If the ferrite grain size of the base metal part is set to 80 μm or less, even if the temperature of the weld heat affected zone is increased by performing normal spot welding, the crystal grains grow much due to the A ₃ transformation point lowering effect described later. Without being maintained at 100 μm or less. As a result, the weld heat affected zone can also maintain a yield stress of 300 N / mm ² or more. The ferrite particle size of the base material portion is more preferably 60 μm or less.
The ferrite grain size can be controlled by adjusting the coiling temperature in hot rolling and an appropriate combination of the final annealing temperatures, as will be described later.

Magnetic permeability in a DC magnetic field of 0.35 Oe: 400 G / Oe or more With respect to magnetic characteristics, if a plated steel sheet having a magnetic permeability in a DC magnetic field of 0.35 Oe is 400 G / Oe or more is used, shielding against geomagnetism The effect is sufficient. By using plated steel plate made of material with high permeability μ _0.35, it prevents geomagnetism from entering the interior of home appliances and electronic devices, suppresses the movement of the electron beam due to geomagnetism, and makes home appliances and electronic devices, etc. Can be operated normally. If the permeability μ _{0.35 of the} plated steel sheet to be used is less than 400 G / Oe, a malfunction due to the influence of geomagnetism may occur in home appliances and electronic devices. For example, color shift occurs in color television.

In the present invention, containing Si, Mn and P amount, to adjust the relationship to satisfy the following equation (1) reduces the A ₃ transformation point, even if the welding heat vicinity portion during spot welding is increased temperature, Phase transformation is performed at a lower temperature so that crystal grains do not grow.
(Si%) × 45− (Mn%) × 40 + (P%) × 150 ≦ 10 (1)
Due to the decrease in the A ₃ transformation point, during the heating, the ferrite grains grow only at a lower temperature, so that significant grain growth can be prevented. Further, during cooling after energization in welding is completed, ferrite transformation occurs at a lower temperature, so that the ferrite grains become finer.
Thus, in order to refine the ferrite grains in the weld heat-affected zone after spot welding, it is effective to lower the A ₃ transformation point by adjusting the steel components, and the details are given in the examples described later. If the value shown in the formula (1) is adjusted to 10 or less by adjusting the amount of addition of Mn and P, the ferrite particle size of the heat affected zone after that is 100 μm even if the steel plate having a ferrite particle size of 80 μm or less is spot welded. It was found that the following can be maintained. Thereby, the yield stress of the weld heat-affected zone after spot welding can be maintained at 300 N / mm ² or more, and a structural component having high plastic deformation resistance can be provided.

Next, a method for producing the steel sheet of the present invention in which the base material has a ferrite grain size of 80 μm or less will be described.
For the production of the steel sheet of the present invention, a general steel sheet production line can be used, and no special process is required. That is, the steel is manufactured in a process of performing normal hot rolling, cold rolling, annealing, and further temper rolling after melting the steel. Cold rolling and annealing can be repeated one or more times depending on the desired plate thickness. However, in order to control the ferrite grain size, the coiling temperature during hot rolling and the final cold rolling It is necessary to adjust appropriately about the combination of the rolling rate of time, and the temperature at the time of final annealing.

Winding temperature during hot rolling: 600 to 700 ° C
Precipitation of AlN is sufficiently advanced in advance at the time of winding, and AlN particles are allowed to grow, whereby fine AlN precipitation that hinders recrystallized grain growth during annealing in the subsequent process can be suppressed. Can be controlled. In order to deposit and grow AlN during winding, it is necessary to wind up at 600 ° C. or higher. If this temperature is not reached, the precipitation and growth of AlN at the time of hot rolling is insufficient, and precipitation occurs at the time of annealing, so that the grains become finer and the magnetic properties are not improved. However, when the coiling temperature exceeds 700 ° C., the scale thickness of the hot-rolled sheet increases, and the deterioration of the surface properties becomes conspicuous. For this reason, in this invention, the coiling temperature after hot rolling is prescribed | regulated to 600-700 degreeC.

Final cold rolling and final annealing:
As described above, in the present invention, AlN is sufficiently precipitated and grown in advance, and then the ferrite grain is finally combined by appropriately combining the rolling rate during the final cold rolling and the temperature during the final annealing. The diameter is controlled in the range of 10 to 80 μm.
In the final cold rolling, a low cold rolling rate is employed to grow ferrite grains coarsely during subsequent annealing. The cold rolling rate affects the grain growth of ferrite grains, and the higher the cold rolling rate, the smaller the ferrite crystal grains tend to be. The magnetic permeability also changes in proportion to the size of the ferrite grains. When the cold rolling rate is less than 10%, it is difficult to recrystallize during annealing, and it becomes difficult to stably obtain ferrite grains of 10 μm or more. Therefore, it is preferable to cold-roll at a rolling rate of 10% or more in the final cold rolling. On the other hand, if the cold rolling rate exceeds 60%, the ferrite grains do not become coarse during annealing, making it difficult to obtain excellent magnetic permeability. For this reason, the cold rolling rate of the final cold rolling is preferably in the range of 10 to 60%.

The ferrite grain size after cold rolling-annealing is more strongly influenced by the annealing conditions at the time of final annealing after the final cold rolling, particularly the annealing temperature, rather than the cold rolling rate at the time of final cold rolling. Therefore, in order to control the ferrite grain size in the range of 10 to 80 μm, the setting of the annealing temperature is important.
In the final annealing after the final cold rolling, as the annealing temperature becomes higher, the coarsening of the ferrite grains proceeds and a high permeability characteristic is obtained. Annealing at a temperature lower than 750 ° C. does not complete recrystallization, and ferrite grains are not sufficiently coarsened, and ferrite grains of 10 μm or more cannot be obtained. However, at an annealing temperature exceeding 900 ° C., the action of recrystallization is saturated, resulting in an increase in manufacturing cost. Therefore, the final annealing is performed in the range of 750 to 800 ° C..
In addition, it is not necessary to prescribe | regulate especially the heat holding time in the last annealing, However It is preferable to set it as about 15 to 120 second in general.
In addition, the combination of the final cold rolling rate and the final annealing temperature can be charted by grasping the relationship between the cold rolling rate and the annealing temperature on the grain size after annealing by repeating experiments in advance. preferable.

Surface plating:
Although the steel plate of the present invention is used as a solid material, it is preferably used in a state where Zn-based plating or Al-based plating is applied for the purpose of improving corrosion resistance. There is no particular limitation on the plating method, and either hot dipping or electroplating may be adopted as long as a ferrite particle diameter within the above range is finally obtained. For example, in hot dipping, Zn plating, Al plating, Zn-4 to 13% Al-1 to 4% Mg plating, etc. can be employed. In electroplating, Zn plating, Zn-10 to 16% Ni plating, etc. can be employed.

When performing hot dip Zn plating or hot dip Al plating, it is preferable to perform “in-line plating” using a continuous annealing line in which the annealing equipment and the plating equipment are integrated. In that case, the annealing performed immediately before plating needs to be the “final annealing” described in this specification. That is, it is possible to employ a method in which the final annealing is performed at an appropriate temperature in the range of 750 to 800 ° C. in the annealing equipment of the continuous hot dipping line, and the steel sheet is immersed in a hot dipping bath during the cooling process.
When performing electro-Zn plating, it is usually performed in a separate line after the final annealing. Electroplating may be performed after temper rolling, or may be performed prior to temper rolling.

Temper rolling:
In order to correct the plate shape, it is effective to perform temper rolling. However, if excessive strain is applied, the magnetic properties deteriorate, so the temper rolling rate is preferably 1.0% or less. In addition, if the temper rolling rate is 1.0% or less, it may be considered that the ferrite grain size does not substantially change before and after the temper rolling.

Example 1: (Influence of steel components)
A steel slab having the composition shown in Table 1 was hot-rolled to a thickness of 3.0 mm under conditions of a hot rolling finishing temperature of 920 ° C. and a winding temperature of 650 ° C., and then cold-rolled to a thickness of 1.2 mm. . Thereafter, the ferrite grain size, tensile properties, and magnetic properties of the steel sheet after continuous annealing at 800 ° C. were investigated.
The ferrite grain size was measured by a cutting method according to JIS G0552 for the cross section including the rolling direction and the thickness direction of the steel sheet.
Tensile properties were investigated in accordance with JIS Z2241 using No. 5 test piece described in JIS Z2201 cut out in parallel to the rolling direction.
Magnetic properties were measured by using a ring-shaped test method described in JIS C2204 for magnetic permeability μ _0.35 which is an index of geomagnetic shielding properties.
On the other hand, the steel plate after annealing was spot welded, and the ferrite grain size of the HAZ part was investigated. The welding conditions were a trapezoidal welding tip with a diameter of 6 mm, a pressure of 3.9 kN, a welding current of 8.0 kA, and a heating time of 35 cycles.
The obtained results are shown in Table 2.

Steel No. which is an example of the present invention. In the case of 1 to 11, the yield stress is 300 N / mm ² or more, and the permeability μ _0.35 is 400 G / Oe or more, and both high strength and high permeability can be achieved. Moreover, the weld heat-affected zone (HAZ zone) and the ferrite grain size at the time of spot welding are as fine as 100 μm or less, and the yield stress is expected to be 300 N / mm ² or more.
On the other hand, in steel Nos. 12 and 13 which are comparative examples, C and N are added in a large amount beyond the specified range, and thus have high strength, but the permeability μ _0.35 is 400 G. / Oe is significantly below. Further, in steel Nos. 14 and 15 which are comparative examples, Si is added in a large amount exceeding the specified range, and thus has a permeability μ _0.35 of 400 G / Oe or more, but ferrite in the HAZ part. The particle size is coarsened to exceed 100 μm. Furthermore, steel No. 16 as a comparative example has a yield strength of less than 300 N / mm ² because the content of P, which is a solid solution strengthening element, is small. Furthermore, steel No. 17 is coarsened so that the ferrite grain size of the HAZ part exceeds 100 μm because the contained Si, Mn and P amounts do not satisfy the formula defined in the present invention.

Example 2: (Spot weld strength)
Using a steel composition of Nos. 8 and 10 in Table 1 whose composition is within the scope of the present invention and No. 14 and 15 outside the scope of the present invention, a shear test of a welded joint specified in JIS Z3136 is performed and spotted. The weld strength was investigated when the welding conditions were changed. The manufacturing conditions of the steel plate are the same as in Example 1. In addition, the spot welding conditions were set to three types in which the welding current was fixed and the applied pressure and the welding current were changed.
Table 3 shows the welding conditions and the survey results.

In the case of the present invention example, the ferrite grain size in the weld heat affected zone (HAZ part) is as fine as 100 μm or less even when the welding conditions change when the structure undergoes phase transformation from ferrite → austenite → ferrite in the process of spot welding. Yes, the breaking strength was high.
On the other hand, in the case of the comparative example, the ferrite grain size of the HAZ part exceeds 100 μm and is coarsened. This is because the A ₃ transformation point rises due to the large amount of Si contained, and the phase transformation from ferrite to austenite does not occur or is not completed. For this reason, even if the tensile strength of a base material part is comparable to this invention steel, the fracture strength of a spot weld part is low.
This tendency is the same even when the welding conditions are changed.

Example 3: (Influence of manufacturing conditions)
Using the steel plate of No. 11 of Table 1 whose component composition is within the scope of the present invention, the relationship between the manufacturing conditions and the material properties was investigated.
The results are shown in Table 4 together with the production conditions.
The No. 1 and 3 steel plates produced under the conditions specified in the present invention can achieve both high permeability μ _0.35 of 400 G / Oe or higher and high yield stress of 300 N / mm ² or higher.
On the other hand, in Nos. 2, 4 and 5, since the annealing temperature is low, the recrystallization of ferrite is not completed, and the magnetic permeability μ _0.35 is low.

Claims (1)

C: 0.003 to 0.010% by mass, Si: 0.5 to 1.0% by mass, Mn: 1.0 to 2.0% by mass, P: 0.04 to 0.15% by mass, Al: Steel slab containing 0.04% by mass or less, N: 0.004% by mass or less, the balance being Fe and inevitable impurities, and the content of Si, Mn and P satisfying the following formula (1) Spot welding excellent in magnetic shielding property, characterized in that the steel sheet is rolled up at a temperature of 600 to 700 ° C. during hot rolling, continuously annealed at a temperature of 750 to 800 ° C. after cold rolling, and then plated. For producing high-strength galvanized steel sheets.

(Si%) × 45− (Mn%) × 40 + (P%) × 150 ≦ 10 (1)