JP2011006764A - Cold rolled steel sheet for projection welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet for projection welding, which has a tensile strength of ≥590 MPa and satisfactory weld zone strength in the case a welding base metal such as a projection nut and a projection bolt is projection-welded, and is suitable as the stock for automotive parts using projection welding.SOLUTION: The cold rolled steel sheet for projection welding has a chemical composition comprising 0.05 to 0.20% C, 0.6 to 2.0% Si, 0.l to 3.0% Mn, ≤0.02% P, ≤0.01% S, ≤1.0% Al and ≤0.01% N, and the balance Fe with impurities, wherein atomic ratio (Si/Mn) is <0.5, and the number density in the surface layer part of the steel sheet from the surface of the steel sheet of Si-Mn based composite oxides with the major axis of ≥50 nm to a depth position of 15 μm is ≤100 pieces/100 μm, the maximum depth of cracks in the surface of the steel sheet is ≤5 μm, also the number density of cracks with a width of ≤6 μm and a depth of ≥2 μm is ≤10 pieces/50 μm, and further, tensile strength is ≥590 MPa.

Description

  The present invention relates to a cold-rolled steel sheet for projection welding. In particular, according to the present invention, a welding nut (hereinafter referred to as a “projection nut”) or a welding bolt (hereinafter referred to as a “projection bolt”) having a high strength and having a projection provided on the seating surface, and the like are provided. The present invention relates to a cold-rolled steel sheet that can be projection-welded with good weld strength.

  Resistance welding typified by spot welding is widely used in various industries including the automobile industry because of low manufacturing costs and equipment costs. A number of techniques have been proposed for spot welding, the most widely used type of resistance welding. These techniques are mainly aimed at obtaining an appropriate nugget diameter in order to ensure good weld strength.

  Incidentally, resistance welding includes, in addition to spot welding, projection welding in which a projection (projection) is provided on a welding base material such as a nut. Projection welding is resistance welding in which current is concentrated and applied to the projections provided on the weld base metal, and is heated and pressed at the same time. By providing projections, the current density can be increased, so the nugget is formed. And a good weld strength can be obtained relatively easily.

  For this reason, the examination about raising the welded part strength in projection welding has not been so far so far, for example, it has been studied at most to devise the projection shape as disclosed in Patent Document 1. Not too much.

JP 2007-218419 A

  However, for example, in order to meet the recent needs for reducing carbon dioxide emissions and improving the safety of automobiles, high-tensile steel materials used in automobile bodies have been promoted. Insufficient strength has become a problem.

  For example, when projection nuts and projection bolts are welded to steel plates, the projection nuts and projection bolts are welded when assembling the welded steel plate members to the car body because the weld strength of projection welding is insufficient. The problem of peeling off at the part and falling off, which hinders assembly work, is spreading.

  The present invention is good in projection welding in order to solve the peeling of the welded portion when a welding base material such as a projection nut or projection bolt, which has become a problem in recent years, is projection welded to a high-strength cold-rolled steel sheet. An object of the present invention is to provide a cold-rolled steel sheet for projection welding that can obtain weld strength.

  In order to solve the above-mentioned problems, the present inventors have examined the cause of the shortage of weld strength in projection welding, which has not been a serious problem in the past, which has become a problem in recent years. As a result, the following new findings were obtained.

(A) The weld strength in projection welding is determined by the nugget portion and the pressure contact portion formed around the nugget.
(B) Since the cold rolled steel sheet having a relatively low strength that has been used for a long time has a low alloy element content, the amount of oxides and the like formed in the steel is small, and the steel is relatively clean. Moreover, since the grain boundary oxidation in the hot rolling process is relatively gradual, cracks are unlikely to occur on the steel sheet surface after pickling and cold rolling. For this reason, the interface of the pressure welding part of projection welding is clean, the presence of voids is suppressed, and a good pressure welding part strength can be obtained.
(C) On the other hand, the relatively high strength cold-rolled steel sheet that has been used in recent years has a high content of alloy elements, so that a large amount of oxides of these alloy elements are formed in the steel. The cleanliness of steel is inferior. Moreover, since grain boundary oxidation is likely to proceed in the hot rolling process, cracks are likely to occur on the steel sheet surface after pickling and cold rolling. For this reason, oxides or the like are likely to intervene at the interface of the press contact portion in projection welding, and further, voids are easily generated, and the strength of the press contact portion is likely to be reduced.
(D) As described above, the cause of the decrease in the strength of the welded portion of projection welding, which has become a problem in recent years, is the decrease in the strength of the pressure welded portion of projection welding accompanying the increase in the strength of the steel plate as the welding base material.

Therefore, in order to solve the above-mentioned problems, the inventors of the present invention have carried out projection welding in accordance with the enhancement of the strength of the steel plate, which is a welding base material, by optimizing from the material surface of the welding base material that has not been studied at all. The method of suppressing the strength reduction of a press-contact part was investigated earnestly. As a result, the following new findings were obtained.
(E) It is a coarse Si—Mn-based composite oxide present mainly at the crystal grain boundary of the steel sheet surface layer portion that intervenes at the interface of the pressure welded portion of projection welding and brings about a decrease in the pressure welded portion strength. Therefore, by restricting the coarse Si—Mn composite oxide present at the crystal grain boundary of the steel plate surface layer portion, it is possible to suppress a decrease in the strength of the pressure welded portion of projection welding.
(F) Further, cracks on the surface of the steel sheet that cause a decrease in the strength of the press-contact portion by forming a void at the interface of the press-contact portion of projection welding have a significant depth or a narrow width and a certain depth. Therefore, by controlling the cracks on the surface of these steel plates, it is possible to suppress a decrease in the pressure welded portion strength of projection welding.

This invention is based on the said new knowledge, The summary is as follows.
In the present invention, C: 0.05% or more and 0.20% or less (in this specification, “%” relating to chemical composition means “mass%” unless otherwise specified), Si: 0.6 %: 2.0% or less, Mn: 0.1% or more, 3.0% or less, P: 0.02% or less, S: 0.01% or less, Al: 1.0% or less, N: 0.01 Si-Mn composite having a chemical composition consisting of Fe and impurities, the atomic ratio of Si and Mn (Si / Mn) being less than 0.5 and the major axis being 50 nm or more The number density in the steel sheet surface layer portion from the oxide steel sheet surface to the 15 μm depth position is 100 pieces / 100 μm ² or less, the maximum depth of cracks on the steel sheet surface is 5 μm or less, and the depth is 6 μm or less. The number density of cracks of 2 μm or more is 10/50 μm or less, and the tensile strength A cold-rolled steel sheet for projection welding, having a mechanical property of 590 MPa or more.

  In the present invention, the chemical composition consists of Cr: 1.0% or less, Mo: 2.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less instead of part of Fe. It is preferable to contain 1 type (s) or 2 or more types selected from the group.

  In these inventions, the chemical composition is selected from the group consisting of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.1% or less, instead of part of Fe. Or it is preferable to contain 2 or more types.

Furthermore, in these inventions, the chemical composition preferably contains Bi: 0.05% or less instead of part of Fe.
The “Si—Mn-based composite oxide” in the present invention is one in which the total content of Si and Mn in the oxide is 30% or more by atomic ratio. In addition, the number density in the steel sheet surface layer portion of the Si—Mn composite oxide having an atomic ratio of Si to Mn (Si / Mn) of less than 0.5 is a plate in the rolling direction that is finish-polished with # 1000 abrasive paper. The thick section is observed at 5000 to 50000 times using SEM, and the composition of the observed major axis of 50 nm or more is analyzed using EDS, whereby the atomic ratio between Si and Mn (Si / Mn). Is obtained by screening the number of Si-Mn based complex oxides in the surface layer portion of the steel sheet from the steel sheet surface to a depth of 15 μm.

  In addition, the “crack on the steel sheet surface” in the present invention is a crack opened on the steel sheet surface, and the measurement of the crack width, depth, and number density is 2000 times using SEM for the above plate thickness cross section. The number density is obtained by observing the observation visual field at 10 arbitrary places where the length in the rolling direction is 50 μm and averaging them.

  According to the present invention, a good weld strength is obtained when a welding base material such as a projection nut or a projection bolt is projection welded while having a high tensile strength of 590 MPa or more, and therefore an automobile in which projection welding is used. A cold-rolled steel sheet suitable as a component material can be provided.

FIG. 1 is an explanatory view schematically showing the shape of the joint surface of the projection nut.

Hereinafter, the invention specific matter of the present invention will be described in detail.
(1) Chemical composition [C: 0.05% or more and 0.20% or less]
C is an element having an effect of increasing the strength of the steel sheet. If the C content is less than 0.05%, it is difficult to ensure a tensile strength of 590 MPa or more. Therefore, the C content is 0.05% or more. On the other hand, if the C content is more than 0.20%, the strength of the welded nugget is increased due to the increase in hardness of the projection weld. Therefore, the C content is 0.20% or less. Preferably it is 0.15% or less.

[Si: 0.6% or more and 2.0% or less]
Si is an element having an action of promoting nugget generation in projection welding by increasing the electrical resistance of steel. Moreover, the atomic ratio (Si / Mn) between Si and Mn of the Si-Mn composite oxide is increased to promote the precipitation of the Si-Mn composite oxide into the crystal grains, and the precipitation to the crystal grain boundaries. Since it suppresses, it also has the effect | action which suppresses the fall of a press-contact part strength. If the Si content is less than 0.6%, the above effect may not be sufficiently obtained. Therefore, the Si content is 0.6% or more. On the other hand, if the Si content is more than 2.0%, the strength of the welded nugget is increased due to the increase in hardness of the nugget in projection welding. Therefore, the Si content is 2.0% or less.

[Mn: 0.1% to 3.0%]
Mn is an element having an effect of increasing the strength of the steel plate. If the Mn content is less than 0.1%, it is difficult to ensure a tensile strength of 590 MPa or more. Therefore, the Mn content is 0.1% or more. On the other hand, if the Mn content is more than 3.0%, the strength of the welded nugget is increased due to the increase in hardness of the nugget in projection welding. Therefore, the Mn content is 3.0% or less.

[P: 0.02% or less]
P is an element contained as an impurity and has an action of segregating in the nugget and reducing the toughness of the nugget portion. When the P content exceeds 0.02%, the toughness of the nugget portion of projection welding is significantly reduced. Therefore, the P content is 0.02% or less.

[S: 0.01% or less]
S is an element contained as an impurity and has an action of reducing the toughness of the nugget portion. Moreover, MnS is formed in steel and the workability of a steel plate is reduced. If the S content exceeds 0.01%, the toughness of the nugget portion in projection welding will be significantly reduced, and the workability of the steel sheet will be significantly reduced. Therefore, the S content is 0.01% or less.

[Al: 1.0% or less]
Al is an element having an action of deoxidizing steel and refining steel in the steel refining process. Moreover, like Si, it is also an element which has the effect | action which accelerates | stimulates the nugget production | generation in projection welding by enlarging the electrical resistance of steel. However, when the Al content exceeds 1.0%, the deterioration of surface properties and workability due to an increase in oxide inclusions become significant. For this reason, Al content shall be 1.0% or less. Preferably it is 0.09% or less. In order to acquire the said effect more reliably, it is preferable to make Al content into 0.005% or more, and it is further more preferable to set it as 0.02% or more.

[N: 0.01% or less]
N is an element contained as an impurity, and has the effect of forming coarse nitrides in the steel and reducing the workability of the steel sheet. If the N content exceeds 0.01%, the workability of the steel sheet will be significantly reduced. Therefore, the N content is 0.01% or less.

[One or more selected from the group consisting of Cr: 1.0% or less, Mo: 2.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less]
These elements are optional elements and have the effect of increasing the strength of the steel sheet by increasing the hardenability of the steel sheet. Therefore, it is preferable to contain one or more of them in order to easily ensure a tensile strength of 590 MPa or more.

However, when the Cr content exceeds 1.0%, the chemical conversion processability is significantly deteriorated. Therefore, the Cr content is 1.0% or less. Preferably it is 0.9% or less.
Even if the Mo content exceeds 2.0%, the Cu content exceeds 1.0%, or the Ni content exceeds 1.0%, the effect of the above action is saturated. This unnecessarily increases the manufacturing cost. Therefore, it is preferable that the Mo content is 2.0% or less, the Cu content is 1.0% or less, and the Ni content is 1.0% or less. More preferably, the Mo content is 1.6% or less, the Cu content is 0.8% or less, and the Ni content is 0.8 or less.

  In order to more reliably obtain the effect of the above action, Cr is 0.1% or more, Mo is 0.05% or more, Cu is 0.05% or more, and Ni is 0.05% or more. It is preferable to make it.

[One or more selected from the group consisting of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.1% or less]
These elements are optional elements, and have the effect of improving the workability of the steel sheet by forming fine precipitates in the steel and refining the crystal grains of the steel sheet. Therefore, in order to ensure better processability, it is preferable to contain one or more kinds.

  However, if the Ti content exceeds 0.05%, the Nb content exceeds 0.05%, or the V content exceeds 0.1%, oxidation of the steel sheet surface layer portion in the hot rolling stage is accelerated. Therefore, cracks in the surface layer portion of the cold-rolled steel sheet may be induced. Therefore, the Ti content is 0.05% or less, the Nb content is 0.05% or less, and the V content is 0.1% or less. More preferably, the Ti content is 0.04% or less, the Nb content is 0.04% or less, and the V content is 0.08% or less.

In order to more reliably obtain the effect of the above action, it is preferable to contain 0.01% or more of Ti, 0.01 or more of Nb, and 0.01% or more of V.
[Bi: 0.05% or less]
Bi is an optional element, and its inclusion makes the solidified structure finer, and even when a large amount of Mn or the like is contained, solidification segregation is suppressed, the structure becomes uniform, and has an action of suppressing deterioration of moldability. Therefore, it is preferable to contain in order to ensure better processability.

However, if the Bi content exceeds 0.05%, the hot workability deteriorates significantly, and hot rolling may be difficult. Therefore, the Bi content is 0.05% or less.
In addition, in order to acquire the effect by the said action more reliably, it is preferable that Bi content shall be 0.0001% or more, and it is more preferable to set it as 0.0010% or more.

The balance other than the above is Fe and impurities.
(2) Si—Mn complex oxide 15 μm depth from the steel sheet surface of the Si—Mn complex oxide whose Si / Mn atomic ratio (Si / Mn) is less than 0.5 and the major axis is 50 nm or more. The number density in the steel sheet surface layer portion up to the position is set to 100 pieces / 100 μm ² or less.

  As described above, it is a coarse Si—Mn-based composite oxide that exists mainly at the crystal grain boundary of the steel sheet surface layer portion that intervenes in the pressure weld interface of projection welding and brings about a decrease in the pressure weld strength. Therefore, by restricting the coarse Si—Mn composite oxide present at the crystal grain boundary of the steel plate surface layer portion, it is possible to suppress a decrease in the strength of the pressure welded portion of projection welding.

  Here, since the Si—Mn composite oxide having an atomic ratio of Si to Mn (Si / Mn) of 0.5 or more is mainly precipitated in crystal grains, it is interposed in the interface of the press contact portion and pressed. Insufficient strength of parts. Therefore, it is necessary to regulate the Si—Mn composite oxide having an atomic ratio (Si / Mn) of Si and Mn of less than 0.5.

  In addition, the Si—Mn-based composite oxide having a major axis of less than 50 nm has little influence on the pressure contact portion strength even if it is interposed at the pressure contact portion interface. Therefore, it is necessary to regulate the Si—Mn composite oxide having a major axis of 50 nm or more.

  In addition, since the Si—Mn-based composite oxide in the central portion of the plate thickness where the depth position from the steel plate surface exceeds 15 μm does not intervene at the interface of the press contact portion, the press contact portion strength is not affected. Therefore, what is necessary is just to regulate the Si-Mn type complex oxide in the steel plate surface layer part from a steel plate surface to a 15 micrometer depth position.

And the number in the steel plate surface layer part from the steel plate surface to the 15 micrometer depth position of the Si-Mn type complex oxide whose atomic ratio (Si / Mn) of Si and Mn is less than 0.5 and whose major axis is 50 nm or more If the density is less than 100 pieces / 100 μm ² , the influence on the pressure contact portion strength is small even if it is interposed at the pressure contact portion interface. Therefore, the number of Si-Mn based complex oxides in the steel plate surface layer portion from the steel plate surface to the 15 μm depth position with an atomic ratio (Si / Mn) of Si and Mn of less than 0.5 and a major axis of 50 nm or more. The density is 100 pieces / 100 μm ² or more.

(3) Cracks on steel plate surface The maximum depth of cracks on the steel plate surface is 5 μm or less, and the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more is 10/50 μm or less.

  As described above, the cracks on the surface of the steel sheet that cause a decrease in the strength of the press-contact portion by forming a void at the press-contact portion interface of projection welding have a significant depth or a narrow width and a certain depth. Therefore, by controlling the cracks on the surface of these steel plates, it is possible to suppress a decrease in the pressure welded portion strength of projection welding.

  If cracks with a depth of 5 μm or more exist on the surface of the steel sheet, or if the number density of cracks with a width of 6 μm or less and a depth of 2 μm or more is more than 10/50 μm, voids are formed at the interface of the pressure welding part of projection welding. Cause a significant decrease in strength of the pressure contact part.

Therefore, the maximum depth of cracks on the steel sheet surface is 5 μm or less, and the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more is 10/50 μm or less.
(4) Tensile strength The cold-rolled steel sheet has a tensile strength of 590 MPa or more. This is because a cold-rolled steel sheet having a tensile strength of less than 590 MPa is less likely to cause the problem of a decrease in weld strength of projection welding, which is an object of the present invention.

(5) Plating layer A plating layer may be provided on the surface of the above-described steel plate for the purpose of improving corrosion resistance, etc., so as to be a surface-treated steel plate. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. Moreover, as the hot dip plating layer, hot dip galvanizing, alloying hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. Illustrated.

(6) Examples of production conditions The cold-rolled steel sheet for projection welding according to the present invention only needs to satisfy the above-mentioned chemical composition, Si-Mn composite oxide, cracks on the steel sheet surface, and tensile strength. The method is not particularly limited, but can be produced, for example, by the following method.

That is, hot rolling is performed on a slab having the above chemical composition set to 1100 ° C. or higher and 1250 ° C. or lower, hot rolling is completed at a finishing temperature of 820 ° C. or higher and 950 ° C. or lower, and winding is performed at 400 ° C. or higher and 600 ° C. or lower. Cooling is performed under the condition that the average cooling rate Vt (° C./s) in the temperature range from the coiling temperature to the temperature T1 (° C.) shown in the following equation (1) satisfies the following equation (2), and further pickling, Apply cold rolling and annealing. Si in this formula (1) indicates the Si content (%), and Al indicates the Al content (%).
T1 = 400−16 × (Si + Al) (1)
Vt> 0.40 (2)
The temperature of the slab to be subjected to hot rolling is preferably set to 1100 ° C. or higher in order to dissolve the alloy element in austenite and to obtain a target steel structure by subsequent heat treatment. On the other hand, it is preferable that the temperature of the slab used for hot rolling is 1250 ° C. or lower from the viewpoint of yield reduction due to scale generation and the cost for increasing the slab temperature.

  The hot rolling completion temperature is preferably set to 820 ° C. or higher in order to suppress ferrite transformation during hot rolling and ensure good workability for the steel sheet after cold rolling and annealing. Moreover, in order to suppress excessive grain growth and to obtain the target mechanical characteristics of the steel sheet after cold rolling and annealing, it is preferable to set the temperature to 950 ° C. or lower.

  The coiling temperature for hot rolling is preferably set to 400 ° C. or higher in order to suppress hardening of the hot-rolled steel sheet, which causes flattening and breakage of the steel sheet in cold rolling. Moreover, in order to suppress the progress of grain boundary oxidation in the hot rolling process that causes cracks on the steel sheet surface after pickling and cold rolling, the temperature is preferably set to 600 ° C. or lower.

  The cooling rate Vt after winding suppresses that Si and Al-based oxides are deposited on the surface layer and cause cracks on the surface of the steel sheet after pickling and cold rolling. When Si becomes an oxide and is removed at the time of pickling, the Si / Mn value of the oxide deposited on the steel sheet surface in the annealing process is reduced, and coarse Si-Mn present in the crystal grain boundary of the steel sheet surface layer portion. In order to suppress the production of the composite oxide, it is preferable to satisfy the above formula (1).

  What is necessary is just to give pickling, cold rolling, and annealing to the hot-rolled steel plate obtained in this way according to a conventional method. The conditions for the cold rolling need not be specified, but the rolling reduction is preferably 38% or more from the viewpoint of obtaining an appropriate texture in order to achieve workability. The annealing conditions do not need to be specified in particular, and may be annealed under appropriate conditions according to the intended tensile strength and workability. For example, annealing is performed at an annealing temperature of 800 ° C. or higher and 850 ° C. or lower.

Furthermore, the present invention will be described more specifically with reference to examples.
The steel having the chemical composition shown in Table 1 is melted and made into a slab by continuous casting, hot-rolled under the conditions shown in Table 2, pickled in a conventional manner, and further under the conditions shown in Table 2. Cold rolling and continuous annealing were performed to obtain various cold rolled steel sheets.

The following test was done about the obtained cold-rolled steel plate.
(A) Tensile test A JIS No. 5 tensile test piece having a longitudinal direction perpendicular to the rolling direction was sampled from various cold-rolled steel sheets, and the tensile properties (yield strength YS, tensile strength TS, total elongation El) were investigated.

(B) Observation of steel plate cross section Si having an atomic ratio (Si / Mn) between Si and Mn of less than 0.5 and a major axis of 50 nm or more in the steel sheet surface layer portion from the steel plate surface to a depth of 15 μm by the method described above. The number density of the -Mn-based composite oxide was measured, and the number density of the Si-Mn-based composite oxide in the steel sheet surface layer portion was determined.

Further, by the above-described method, the presence / absence of cracks having a maximum depth exceeding 5 μm and the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more were obtained.
(C) Projection welding 60 mm × 60 mm test specimens were taken from various cold-rolled steel sheets, a hole with a diameter of 11 mm was drilled in the center, and a flanged M10 welding nut having the shape of the joint surface shown in FIG. ) Was set in an AC welding machine so that the center of the hole of the test piece and the center of the hole of the nut coincided, and projection welding was performed under the welding conditions shown in Table 3.

  After fixing the bolt to the nut hole of the welded body thus obtained, the load when the nut peeled from the steel sheet was measured by the indentation peeling test method defined in JIS B 1196: 2001. The test results are shown in Table 4.

Sample No. in Table 4 Nos. 11 and 14 to 20 are examples of the present invention. 1-10, 12 and 13 are comparative examples.
Sample No. of Example of the present invention. As for 11 and 14-20, as for all, the favorable weld part strength whose peel strength is 6.5 kN or more was obtained.

  On the other hand, the test sample No. 1 to 4 and 6 to 10 have a high Ti content, and therefore there are many cracks having a width of 6 μm or less and a depth of 2 μm or more, which causes a decrease in strength of the press contact portion and a low peel strength.

  Sample No. of Comparative Example Nos. 1 to 8 have a low Si content, so that the Si—Mn composite oxide having an Si / Mn atomic ratio (Si / Mn) of less than 0.5 and a major axis of 50 nm or more is present in the steel sheet surface layer portion. A large amount of the material was present, resulting in a decrease in strength of the press contact portion, and further, the nugget generation during welding was insufficient, so that the peel strength was low.

  Sample No. of Comparative Example No. 12, because the hot rolling coiling temperature was high, cracks with a depth of more than 5 μm existed on the steel sheet surface, and many cracks with a width of 6 μm or less and a depth of 2 μm or more existed on the steel sheet surface. The peel strength was low.

  Furthermore, the test material No. No. 13 had an inappropriate cooling rate after winding, so that there were cracks with a depth of more than 5 μm on the surface of the steel plate and many cracks with a width of 6 μm or less and a depth of 2 μm or more on the steel plate surface. And the Mn atomic ratio (Si / Mn) is less than 0.5 and the major axis is more than 50 nm Si-Mn based complex oxides are present in the surface layer of the steel sheet, reducing the strength of the pressed part. Invited, peel strength was low.

Claims (4)

In mass%, C: 0.05% to 0.20%, Si: 0.6% to 2.0%, Mn: 0.1% to 3.0%, P: 0.02% or less S: 0.01% or less, Al: 1.0% or less, N: 0.01% or less, and the balance having a chemical composition consisting of Fe and impurities,
The number density in the steel sheet surface layer portion from the steel plate surface to the 15 μm depth position of the Si—Mn based complex oxide having an atomic ratio (Si / Mn) of Si and Mn of less than 0.5 and a major axis of 50 nm or more. 100/100 μm ² or less,
The maximum depth of cracks on the steel sheet surface is 5 μm or less, and the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more is 10/50 μm or less,
A cold-rolled steel sheet for projection welding, having mechanical properties of a tensile strength of 590 MPa or more.   The chemical composition is selected from the group consisting of Cr: 1.0% or less, Mo: 2.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less, instead of a part of the Fe. The cold-rolled steel sheet for projection welding according to claim 1, comprising one or more kinds.   The chemical composition is one or more selected from the group consisting of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.1% or less, instead of a part of the Fe. The cold-rolled steel sheet for projection welding according to claim 1 or 2, characterized by comprising:   The cold rolled steel sheet for projection welding according to any one of claims 1 to 3, wherein the chemical composition contains Bi: 0.05% or less instead of a part of the Fe.

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