JP3454019B2 - Hot rolled steel sheet with excellent flash weldability and upset weldability and non-oriented electrical steel sheet made from this hot rolled steel sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet having excellent flash weldability and a non-oriented electrical steel sheet produced from this hot-rolled steel sheet.

[0002]

2. Description of the Related Art Non-oriented electrical steel sheets are used for generators, electric motors,
It is used in a wide range of electrical equipment such as small and medium-sized transformers.

Non-oriented electrical steel sheets are subjected to finish annealing, etc. by a steel sheet maker, and are fully processed products already having final magnetic properties when shipped from the steel sheet maker, and steel sheets after finish annealing. On the other hand, after processing such as punching and shearing at the customer, magnetic annealing is performed,
There is a semi-processed product that is designed to remove the processing strain and coarsen the crystal grains so as to obtain predetermined magnetic characteristics.

Usually, a non-oriented electrical steel sheet is obtained by hot rolling a steel slab melted in a converter or an electric furnace and then pickling it, followed by hot-rolled sheet annealing or hot-rolled sheet annealing. It is manufactured by performing cold rolling-annealing, or two or more cold rollings with intermediate annealing sandwiched between them, and finish annealing without performing. If necessary, magnetic annealing is further performed after punching and shearing.

In the production of the above-mentioned non-oriented electrical steel sheet, the hot-rolled steel sheet is usually joined by a welding machine installed on the inlet side of the pickling line, and passed through each subsequent production line. , Predetermined processing is performed.

In the production line, the welded portion is treated in the same manner as the normal portion of the steel sheet other than the welded portion. Therefore, when the weld strength is low, the weld breakage is likely to occur in the production line, particularly in the cold rolling line where a large rolling load and tension are applied. When a weld rupture occurs, it causes a large decrease in manufacturing efficiency and a large increase in manufacturing cost due to equipment damage such as rolling roll damage due to the ruptured material. Therefore, it is necessary that the welded portion has such strength that it does not break within the manufacturing line described above.

Various welding methods are used for joining hot-rolled steel sheets. Among them, flash welding and upset welding are less costly than equipment such as laser welding.
The running cost is low. However, when welding steel plates containing alloy components that easily form inclusions such as Si and Mn by these methods, inclusions remain at the weld interface, which causes cracking, which causes the weld strength. Is easy to decrease.

Because of its relatively high efficiency, flash welding is often used to connect the preceding and following steel plates on the entry side of the pickling line. Therefore,
The case of flash welding will be described.

Flash welding is a welding method in which a welded portion is formed and connected to a material to be welded by the following three steps.

With a voltage applied to both ends of the material to be welded,
The abutting end faces are brought close together at an appropriate speed to create a local contact.

The end face is locally concentratedly heated by resistance heat generation due to a high-current-density short-circuit current flowing in the contact portion and an arc generated by melting and breaking of the contact short-circuit portion. At this time, the arc causes the molten metal on the end surface to scatter. This scattering phenomenon of molten metal is called flushing. (Flush process) When the temperature of the entire end face is increased by flushing and a substantially uniform molten layer is formed on the entire abutting end face, the material to be welded is rapidly advanced to pressurize and deform the welded part. , Welding materials to be welded together. (Upset process)

As described above, when a steel sheet containing an alloy component that easily forms inclusions such as Si and Mn is welded, the inclusions remain at the weld interface, and the inclusions cause cracks to occur.
The weld strength is likely to decrease. Since the non-oriented electrical steel sheet contains many alloy components that are likely to generate inclusions such as Si and Mn added to improve the electromagnetic characteristics, the hot rolled steel sheet used for manufacturing the non-oriented electrical steel sheet is flashed. When welding by welding, it is important to avoid the inclusion of inclusions at the weld interface due to the alloy components.

As a method of avoiding the inclusions from remaining at the weld interface, first, the welding conditions are optimized so that the inclusions can be completely discharged from the weld interface in the upset process, and strictly controlled and implemented. There are things to do. However, in the case of a steel sheet containing a large amount of alloying elements that easily form inclusions, the proper welding condition range itself does not exist, or the condition range is significantly narrowed even if it exists. Even if there is a proper welding condition range, the complicated welding conditions such as the magnitude of flash current and flash time, the magnitude of upset current and upset time, and many factors such as flash allowance and upset allowance are entangled. Strict control and stable operation are extremely difficult industrially.

Therefore, a method has been proposed in which the formation of inclusions during welding, particularly the formation of oxides that are most easily formed in the inclusions, is suppressed and the inclusions are prevented from remaining.

Japanese Unexamined Patent Publication No. 49-96947 discloses a method of cutting off oxygen in the atmosphere by burning a gas containing a reducing gas in the vicinity of a welded portion. A method has been proposed in which the welded part is hermetically sealed, shielded with an inert gas, and welded.

Further, in Japanese Patent Laid-Open No. 59-118282, a protective gas such as a carbon polymer, an organic compound, an organic silicon compound, zinc, zinc powder inclusions, calcium, magnesium alloys or their inclusions is provided near the welded portion. Generated substances are disclosed in JP-A-62-27558
JP-A No. 63-203281 proposes a method of applying grease to the vicinity of each welded part and generating a protective gas by heat during welding, and JP-A-63-203281 discloses a method of applying grease. .

[0017]

However, the technique proposed above has the following problems.

Since the technique proposed in Japanese Patent Laid-Open No. 49-96947 utilizes gas combustion, there is a problem that the cost increases due to the installation and operation of gas combustion equipment, and the thermal influence on the peripheral equipment due to the flame increases. However, it is not practical. JP-A-56-5
The technology proposed in the 0789 publication has a problem in that the cost is increased due to equipment modification and its operation due to airtightness, and that maintenance work around the butt portion that requires delicate adjustment by attaching these equipment to the welding machine will be hindered. Occurs.

Furthermore, when a large amount of alloying elements having a strong oxidizing property such as Si and Mn are contained, inclusions such as oxides may remain at the weld interface, so that the weld strength is not always sufficient. Can't get

JP-A-59-118282 and JP-A-62-275581
The technique proposed in Japanese Patent Publication No. 63-203281 can relatively easily widen the range of appropriate welding conditions.
The effect of shielding the surroundings of the weld during welding from the atmosphere is not always perfect, and if a large amount of alloying elements with strong oxidizing properties such as Si and Mn are contained, inclusions such as oxides will remain at the weld interface. However, it is not always possible to obtain sufficient reliability regarding the strength of the weld.

Further, the number of steps for applying grease or the like to the steel sheet increases, which causes a decrease in efficiency and an increase in equipment cost.

Further, when these methods are used, grease or the like adheres to and remains on the welding machine or the like, and spatter or the like tends to adhere. When the spatter adheres to the surface of the electrode, the contact between the electrode and the steel sheet is localized, and welding defects such as die burn occur. Further, if spatter falls on the steel sheet during passage of the steel sheet, the surface of the steel sheet will be damaged, so that complicated cleaning for removing grease and the like must be performed. For the above reasons, these are also methods that we want to avoid as much as possible.

Therefore, in an actual production line, in the case of a difficult-welding material such as a non-oriented electrical steel sheet material that contains a large amount of Si and the like and contains an alloy component that easily forms inclusions due to oxides, oxidization is difficult. It is also practiced to insert a mild steel strip or the like, which does not contain a large amount of constituent elements, between difficult-to-weld materials, weld them, and alternately pass through them.

The present invention has been made in consideration of the above-mentioned circumstances, and welding is performed in the production line without applying grease or the like and alternately passing through a material that does not contain a large amount of oxidizing component elements. It is an object of the present invention to provide a hot-rolled steel sheet having excellent flash weldability and upset weldability capable of obtaining a weld strength that does not cause part breakage, and a non-oriented electrical steel sheet produced using this hot-rolled steel sheet as a raw material.

[0025]

The features of the present invention for solving the above problems are as follows.

The first aspect of the present invention is, by weight%, Si: 1.0.
% Or less, Al: 0.004% or less, Mn: 0.1 to 0.
A steel sheet containing 8%, P: 0.2% or less, S: 0.02% or less, C: 0.005% or less, N: 0.005% or less, and the balance being Fe and inevitable impurities. , 50 inclusions having a diameter of 0.1 to 1.0 μm contained in the steel sheet
40 inclusions with a diameter of less than 00 / mm ² and a diameter of more than 1.0 μm
It is a hot-rolled steel sheet excellent in flash weldability and upset weldability of 0 pieces / mm ² or less.

The second aspect of the present invention is, by weight%, Si: 1.0.
% Or less, Al: 0.004% or less, Mn: 0.1 to 0.
A steel sheet containing 8%, P: 0.2% or less, S: 0.02% or less, C: 0.005% or less, N: 0.005% or less, and the balance being Fe and inevitable impurities. , The inclusions in the steel sheet having a diameter of 0.1 to 1.0 μm are 30
40 inclusions with a diameter of less than 00 / mm ² and a diameter of more than 1.0 μm
It is a hot-rolled steel sheet excellent in flash weldability and upset weldability of 0 pieces / mm ² or less.

The third invention is, in wt%, Si: 1.0.
% Or less, Al: 0.004% or less, Mn: 0.1 to 0.
A steel sheet containing 8%, P: 0.2% or less, S: 0.02% or less, C: 0.005% or less, N: 0.005% or less, and the balance being Fe and inevitable impurities. , The inclusions in the steel sheet having a diameter of 0.1 to 1.0 μm are 30
20 inclusions with a diameter of less than 00 / mm ² and a diameter of more than 1.0 μm
It is a hot-rolled steel sheet excellent in flash weldability and upset weldability of 0 pieces / mm ² or less.

A fourth invention is a non-oriented electrical steel sheet produced by flash-welding or upset-welding the hot-rolled steel sheet of the first to third inventions, and then cold rolling and annealing at least once. .

The present invention will be described below. The inventors of the present invention, when flash-welding a hot-rolled steel sheet containing a large amount of strongly oxidizing alloy elements such as Si and Mn, even if the generation of oxides during welding is suppressed, inclusions remain at the weld interface. Focusing on the fact that the strength of the welded part may decrease due to this, the relationship between the steel plate composition and inclusions in the steel plate and flash weldability was investigated in detail.

As a result, the flash weldability is not directly affected by the composition of the steel sheet, but is greatly affected by the inclusions in the steel sheet. Furthermore, as will be described later, the inclusions in the steel sheet have a large size. It was found that the effect on the flash property was different depending on the size.

Based on this knowledge, the present invention is characterized in that it is possible to obtain good flash weldability and upset weldability by setting the size and number of inclusions in the steel sheet within an appropriate range. is there.

The above points will be described in detail below. First, the composition of the composition shown in Table 1 thickness 2.8 ~ 3.0mm, width 1400mm
The hot-rolled steel sheets of No. 1 were butted and flash-welded. The rest of the composition of the steel sheet in Table 1 not shown is Fe and inevitable impurities. The welding conditions at this time are: flash time 8.5 seconds, flash length 12 mm, upset cost 3.5 mm
Is. The flash metal discharged from the welded part was ground by a cutting tool to the same height as the base metal surface.

Flash weldability was evaluated by the mechanical quality of the weld. The mechanical quality of the welded part was obtained by taking out 25 pieces of a 50 mm wide test piece from the 1400 mm wide welded part, performing a 180 degree bending test with a 6R punch, and measuring the crack length generated in the welded part after the bending test, The ratio of the crack length to the entire length of the welded portion was determined as a crack length ratio (%, crack length (mm) / 50 × 100), and evaluated by this value. Table 1 also shows the results of the crack length ratio survey.

In a normal pickling line or cold rolling line, if the crack length ratio is 10% or less, a welded part is not broken in the line and stable striping is possible. .

[0036]

[Table 1]

Although the crack length ratio tends to increase as the Si content increases, the crack length ratio varies considerably even for steel sheets having almost the same composition. Therefore, since it is not possible to judge whether the flash weldability is good or not only by the steel plate component composition, it was found that the flash weldability cannot be improved only by limiting the steel plate component composition.

After further examination, the following was found from the results of observation of inclusions in the steel sheet before and after welding and the results of investigation of weldability.

Equivalent circle diameter (diameter, same below) 1.0 μm
At the time of flash welding of super inclusions, most of these inclusions are melted in the molten metal, but they are not melted in the vicinity of the melting interface, and conversely grow due to the temperature rise. The growth of such inclusions becomes more significant as it approaches the melting interface.

The molten metal itself is relatively easily discharged if the upsetting process is properly performed. However, inclusions grown in the vicinity of the melt interface are difficult to discharge during upset, and remain at the weld interface to become the starting point of cracking.

During flash welding of inclusions of 0.1 to 1.0 μm, these inclusions are almost completely melted in the molten metal. In the vicinity of the melting interface, there are some that agglomerate and become larger, but most of them melt due to the temperature rise during welding.

The molten metal itself is relatively easily discharged if the upsetting process is properly performed. However, the inclusions melted in the vicinity of the melt interface finely regenerate at the weld interface at the time of quenching after the completion of welding to form a flat spot. If this flat spot widely exists at the weld interface, the interface strength will decrease. In particular, when a crack occurs, it facilitates its propagation.

Therefore, by defining the size and amount of these inclusions, it is possible to perform welding in the manufacturing line without applying grease or the like and alternately passing through a material that does not contain a large amount of oxidizing component elements. It was thought that a steel plate with more stable flash weldability capable of obtaining a weld strength that does not cause fracture can be obtained.

Based on such knowledge, the relationship between the amount and size of inclusions and flash weldability was continuously investigated.

In order to investigate the influence of inclusions, steel plates A and B having the composition shown in Table 2 were manufactured as follows. The balance of the composition of the steel sheet in Table 2 not shown is Fe and inevitable impurities.

[0046]

[Table 2]

The inclusions present in the steel sheet are divided into a primary deoxidation product produced in the melting stage of the molten steel and a secondary deoxidation product crystallized due to the decrease in solubility during solidification cooling. Therefore, using a vacuum melting furnace to adjust the basicity and composition of the slag, adjust the degassing time, and adjust the cooling rate during solidification of molten steel, the steel melted to the composition shown in Table 2 A slab with the size and number of products adjusted was manufactured. These slabs were hot-rolled to produce a sample having a plate thickness of 2.0 mm and a plate width of 110 mm.

Inclusions were observed and flash welding was performed on this sample. After welding, a sample with a width of 50 mm was cut out and the mechanical quality was evaluated by the bending test / evaluation method described above. Regarding the observation of inclusions, the observation of inclusions with a diameter of more than 1.0 μm was performed with an optical microscope, and the observation of inclusions with a diameter of 0.1 to 1.0 μm was performed with a scanning electron microscope.

FIG. 1 shows the results of investigation on the relationship between the number of inclusions having a diameter of more than 1.0 μm and the crack length ratio for the steel plate A. In this case, the number of inclusions of 0.1 to 1.0 μm is almost constant at 700 to 1500 / mm ² .

As is clear from FIG. 1, the crack length ratio tends to decrease with a decrease in the number of inclusions having a diameter of more than 1.0 μm. By setting this number to 400 / mm ² or less, the crack length can be reduced. The slab ratio is 10% or less, which is within the allowable range when passing through the line. When the number is 200 pieces / mm ² or less, the crack occurrence rate is further low and stable.

FIG. 2 shows the results of an investigation conducted on the steel plate B in the same manner as the case of the steel plate A described above. Steel plate B
For steel sheet A, a tendency similar to that of steel plate A in Fig. 1 was observed, and by setting the number of inclusions with a diameter of more than 1.0 μm to 400 pieces / mm ² or less, the crack length ratio became 10% or less. There is.
Further, as in the case of FIG. 1, when the number is 200 pieces / mm ² or less, the crack occurrence rate is further low and stable.

From the above results, in the present invention, the number of inclusions exceeding 1.0 μm is 400 pieces / mm ² or less, more preferably 200 pieces / mm ^2.
Limited to mm ² or less.

Next, the number of inclusions having a diameter of more than 1.0 μm is 400 /
An examination was conducted in the case where the number of inclusions was changed to 0.1 to 1.0 μm with the value being less than or equal to mm ² .

FIG. 3 shows the number of inclusions and the crack length when the number of inclusions having a diameter of more than 1.0 μm is 400 pieces / mm ² or less and the number of inclusions of 0.1 to 1.0 μm is changed. It shows the relationship of the rates. Further, FIG. 4 also shows a similar relationship for the steel plate B.

As shown in FIG. 3 and FIG.
The crack length ratio is 10% or less when the number of inclusions of 1.0 μm is 5000 pieces / mm ² or less, and is more stable and excellent when the number of inclusions is 3000 pieces / mm ² or less.

Therefore, in the present invention, the diameter is 0.1 to 1.0 μm.
Limit the number of m inclusions to 5000 / mm ² or less. A more preferable range is 3000 pieces / mm ² or less.

Next, the reasons for limiting the chemical components of the present invention will be described. Si: A component effective in increasing the specific resistance of the steel sheet and reducing iron loss, but if it exceeds 1.0%, the magnetic flux density decreases,
The cost is high, so the upper limit is 1.0%.

Al: 0.004% or less because the growth of crystal grains is inhibited and the iron loss is increased by forming fine AlN.

Mn: A component effective in increasing the specific resistance of the steel sheet to reduce iron loss, and is required to be 0.1% or more in order to prevent red hot embrittlement during hot rolling. However, excessive addition lowers the saturation magnetic flux density, so the upper limit is made 0.8%.

P: It is a component necessary for improving the punchability of the steel sheet, but if it is added in excess of 0.2%, the steel sheet becomes brittle, so it is made 0.2% or less.

S: Since it promotes the formation of sulfides such as MnS which causes cracks along the metal flow of the welded portion and deteriorates magnetic properties, it is preferable that the content is as low as possible, but 0.02 from the viewpoint of manufacturing cost. % Or less.

C: 0.005% or less because it is a harmful component that increases iron loss and causes magnetic aging.

N: AlN that inhibits the growth of crystal grains during annealing
Of 0.005% or less to promote the formation of iron and increase the iron loss.

[0064]

BEST MODE FOR CARRYING OUT THE INVENTION
Any form may be used as long as both the number of inclusions having a diameter of more than 1.0 μm and the number of inclusions having a diameter of 0.1 to 1.0 μm are adjusted within a predetermined range.

For example, molten steel melted in a predetermined range of components in a vacuum melting furnace, a converter or an electric furnace is subjected to degassing treatment, etc., and cast by ingot casting, continuous casting or strip caster, and in a hot piece state. Direct hot rolling or hot working after reheating the billet, in the production of hot rolled steel sheet, the inclusion of a primary deoxidation product having a diameter of more than 1.0 μm is mainly controlled, for example, molten steel degassing time. Or to prevent reoxidation from the slag by adjusting the slag composition.Furthermore, the inclusion of mainly secondary deoxidation products with a diameter of 0.1 to 1.0 μm is controlled by the casting speed. It can be performed by adjusting the solidification cooling rate of molten steel such as heating during casting, auxiliary heating, heat retention, slab thickness, and cooling conditions.

As for S, it is desirable to remove S by hot metal, ladle refining, or the like, or to refine by using a raw material containing a small amount of S or an auxiliary raw material.

For hot working, finish hot rolling is indispensable among slab rolling, rough rolling and finish hot rolling, but slab rolling and rough rolling are performed on steel ingots, billets, cast plates, etc. after casting. It can be appropriately selected depending on the thickness dimension, suppression of ridging and the like.

Since the hot-rolled steel sheet produced in this manner has excellent flash weldability and upset weldability, the hot-rolled steel sheet is used as it is, or after being subjected to descaling such as pickling, etc. It can be widely used for applications of hot-rolled steel sheets that undergo flash welding and upset welding.

The hot-rolled steel sheet may be processed by connecting the preceding steel sheet and the following steel sheet by flash welding or the like and then passing them through each production line such as a pickling line or a cold rolling line. Since it does not break in the welded portion, it can be used as a material for various cold-rolled steel sheets, but is particularly preferably used as a material for non-oriented electrical steel sheets.

The non-oriented electrical steel sheet is manufactured as follows. That is, the hot-rolled steel sheet is manufactured by performing flash welding or upset welding on the entry side of the pickling line, pickling, and then cold rolling and annealing at least once. Hot-rolled sheet may be annealed before cold rolling. The cold rolling and the annealing may each be performed once, or the cold rolling and the finish annealing may be performed twice or more with the intermediate annealing interposed therebetween. Further, if necessary, magnetic annealing may be performed after punching or shearing. In this way, a predetermined full process or semi-processed non-oriented electrical steel sheet is manufactured through a process capable of obtaining the properties required for the non-oriented electrical steel sheet.

Since the non-oriented electrical steel sheet produced in this manner does not have a weld fracture in each production line following pickling, it is produced due to a reduction in production efficiency due to fracture and equipment damage such as rolling roll damage. The non-oriented electrical steel sheet has the required magnetic properties without increasing the cost.

[0072]

EXAMPLE Slabs were produced under various refining-casting conditions. That is, the size and the number of inclusions were adjusted by changing the basicity by adjusting the slag composition, changing the vacuum degree and degassing time, and changing the solidification cooling rate of the molten steel during casting.

The slab thus obtained was heated to 1200 ° C. and then hot-rolled into a hot-rolled steel sheet having a plate thickness of 3.0 mm and a plate width of 1400 mm, which was welded by a flash welding machine. The welding conditions are a flash time of 8.5 seconds, a flash length of 12 mm, and an upset margin of 3.5 mm. The flash metal discharged from the welded part was ground by a cutting tool to the same height as the base metal surface.

Flash weldability was evaluated by the mechanical quality of the weld. The mechanical quality of the welded part was obtained by taking out 25 pieces of a 50 mm wide test piece from the 1400 mm wide welded part, performing a 180 degree bending test with a 6R punch, and measuring the crack length generated in the welded part after the bending test, The ratio of the crack length to the entire length of the welded portion was determined as a crack length ratio (%, crack length (mm) / 50 × 100), and evaluated by this value.

The chemical composition of the steel sheet and the survey results are summarized in the table.
Shown in 3. The rest of the chemical composition in Table 3 is not shown.
Fe and inevitable impurities.

[0076]

[Table 3]

All the steel sheets of the present invention have a crack length ratio of 10%.
It is below, and the flash weldability is very good.

Inventive Example 2 that satisfies the configuration of the second invention
The steel sheet of No. 2 has a lower crack occurrence rate and is more excellent in flash weldability than the steel sheet of Inventive Example 1 which does not satisfy the configuration of the second invention.

Furthermore, an example of the invention satisfying the constitution of the third invention
The steel sheet of No. 3 has a lower crack occurrence rate than the steel sheets of Invention Example 1 and Invention Example 2 which do not satisfy the constitution of the third invention, and is excellent in flash weldability.

Further, the diameter is 0.1 to 1.0 μm, or the diameter is 1.
The steel sheet of the comparative example in which the number of inclusions exceeding 0 μm is out of the range of the present invention has a crack occurrence rate of more than 10% and is inferior in flash weldability.

In the above description, only flash welding has been described, but similar results can be obtained in upset welding without a flash process.

[0082]

As described in detail above, the hot-rolled steel sheet of the present invention has good flash weldability without applying grease or the like, and passing through with a material not containing a large amount of oxidizing component elements. And upset weldability. Therefore, it can be widely used for hot-rolled steel sheets for flash welding and upset welding. Also, when hot-rolled steel plates connected by flash welding or upset welding are processed by passing them through each manufacturing line such as a cold rolling line, there is no rupture of the welded part in each manufacturing line. Does not cause increase in manufacturing cost due to equipment damage such as deterioration of rolling stock and damage to rolling rolls. Further, the methodless electromagnetic steel sheet manufactured using this hot-rolled steel sheet as a material can obtain good required electromagnetic characteristics in addition to the above-mentioned effects.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the number of inclusions having a diameter of more than 1.0 μm in steel plate A and the crack length ratio after flash welding.

FIG. 2 is a diagram showing the relationship between the number of inclusions having a diameter of more than 1.0 μm in steel plate B and the crack length ratio after flash welding.

FIG. 3 shows the number of inclusions having a diameter of 0.1 to 1.0 μm in the steel plate A,
It is a figure which shows the relationship of the crack length rate after flash welding.

FIG. 4 shows the number of inclusions having a diameter of 0.1 to 1.0 μm in the steel plate B,
It is a figure which shows the relationship of the crack length rate after flash welding.

Front page continuation (72) Inventor Akira Hirano 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe Co., Ltd. (72) Yoshihiko Oda 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe Co., Ltd. (72) Inventor Kentaro Ikejiri 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) Reference JP-A-60-245768 (JP, A) JP-A-5-255801 (JP, A) ) JP-A-4-168217 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 303 C21D 8/12 C22C 38/06

Claims (4)

(57) [Claims] 1. By weight%, Si: 1.0% or less, Al:
0.004% or less, Mn: 0.1 to 0.8%, P: 0.
Contains 2% or less, S: 0.02% or less, C: 0.00
5% or less, N: 0.005% or less, balance Fe and
A steel sheet made of unavoidable impurities, wherein the number of inclusions having a diameter of 0.1 to 1.0 μm contained in the steel sheet is not more than 5000 / mm ² , and the number of inclusions having a diameter exceeding 1.0 μm is not more than 400 / mm ² . A hot-rolled steel sheet excellent in flash weldability and upset weldability, which is characterized by being present. 2. Si: 1.0% or less by weight, A
1: 0.004% or less, Mn: 0.1 to 0.8%, P:
0.2% or less, S: 0.02% or less, C: 0.
005% or less, N: 0.005% or less, the balance Fe and
And unavoidable impurities, and the number of inclusions in the steel sheet having a diameter of 0.1 to 1.0 μm is 3000 pieces / mm.
^2, hot-rolled steel sheet excellent in the flash weldability and upset weldability, wherein the inclusions with a diameter of 1.0μm greater is 400 / mm ² or less. 3. By weight%, Si: 1.0% or less, A
1: 0.004% or less, Mn: 0.1 to 0.8%, P:
0.2% or less, S: 0.02% or less, C: 0.
005% or less, N: 0.005% or less, the balance Fe and
And unavoidable impurities, and the number of inclusions in the steel sheet having a diameter of 0.1 to 1.0 μm is 3000 pieces / mm.
A hot-rolled steel sheet excellent in flash weldability and upset weldability, characterized in that the number of inclusions having a diameter of ² or less and a diameter of more than 1.0 μm is 200 pieces / mm ² or less. 4. A non-oriented electrical steel sheet produced by subjecting the hot-rolled steel sheet according to claim 1 to flash welding or upset welding, followed by cold rolling and annealing at least once.