JPH07138699A - Steel plate, excellent in spot weldability and surface characteristic, and its production - Google Patents

Abstract

PURPOSE:To obtain a steel plate excellent in spot weldability and surface characteristic by specifying a composition consisting of C, N, Mn, Si, P, S, Al, and Ti and relations among these components, respectively. CONSTITUTION:This steel plate has a composition containing, by weight, <=0.005% C, <=0.004% N, 0.03-2.7% Mn, <=0.8% Si, <=0.1% P, 0.001-0.02% S, 0.01-0.08% sol.Al, and 0.005-0.12% Ti, further containing, if necessary, 0.0002-0.003% B and further 0.003-0.05% Nb, and satisfying (Ne/14)+(S/32)<=0.00023 and (Mn+Tie)/S>=8, where Ne=min[(14Ti/48), N] and Tie-Ti--(48Ne/14). This steel plate has excellent spot weldability and surface characteristic. This steel plate can be produced by continuously casting a steel of the above composition, temporarily cooling the part, between the surface and a position at a depth of 50mm as a minimum and a position at a depth of 20% of the thickness of cast slab as a maximum, of the resulting cast slab down to a temp. not higher than the Ar1 point, recuperating the surface of the cast slab to 1000-1150 deg.C again, and immediately applying hot rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet suitable for applications requiring good spot weldability such as automobiles, that is, a hot rolled sheet, its surface treated steel sheet, cold rolled steel sheet, and its surface treatment. TECHNICAL FIELD The present invention relates to a steel plate and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, as a measure for producing a steel sheet having high press formability, Japanese Patent Publication No. 44-18066, Japanese Patent Publication No. 54-1245, and Japanese Patent Laid-Open No. 59-67.
So-called IF disclosed in Japanese Patent No. 319, etc.
Steel, that is, steel in which nitrogen and carbon in the steel are reduced as much as possible and carbonitride forming elements such as Ti and Nb are added, has been widely used.

However, since the IF steel is an ultra-low carbon steel, the crystal grains in the spot-welded heat-affected zone are likely to become coarse, and the strength of the spot-welded joint is low. Therefore, Japanese Patent Laid-Open No. 3-177538, which regulates the amount of oxygen in steel,
JP-A-63-317625 which defines the amount of Ti-Nb-B
JP-A-63-317648, JP-A-63
There is Japanese Patent Application Laid-Open No. 63-317647 and Japanese Patent Application Laid-Open No. 63-317647 which defines the size and amount of Ti precipitates.

These methods are basically premised on a process of reheating a slab once cooled to room temperature below the A ₃ transformation point and hot rolling. On the other hand, from the viewpoint of energy saving and process saving, a method of directly feeding and rolling an IF steel without cooling a continuously cast slab to a temperature not higher than the A ₃ transformation point has been disclosed (for example, JP-A-59-1).
23721, and JP-A-62-278232).

However, such a mere straight rolled material has a problem that surface cracks are likely to occur during hot rolling and the yield is reduced. Therefore, although not limited to IF steel, JP-A-59-189001, JP-A-2-37950, and JP-B-2-18936 intend only the surface layer portion of a continuously cast slab from the inside. Discloses a direct-rolling method in which the susceptibility to surface cracking is reduced by mechanical cooling.

[0006]

However, the above-mentioned prior art has the following problems. The method of cooling the continuously cast slab to room temperature below the A ₃ transformation point and reheating and rolling is problematic in terms of energy saving and process saving, and the method of direct feed rolling is problematic in surface cracking. In addition, the method of cooling only the surface layer of continuous cast slabs so far does not target ultra-low carbon steel, but focuses only on surface crack countermeasures, and actively improves the material quality. It's not something you want to do.

Further, the method of defining the size and amount of Ti precipitates requires a casting speed that cannot be achieved by ordinary continuous casting, and does not attempt to control the surface layer portion and the inside precipitates separately. Therefore, the present invention is based on a method of strongly cooling only the surface layer portion of a continuously cast slab of IF steel, which has not been paid attention in the past, and then recovering heat by sensible heat inside the slab. To provide an IF steel sheet having positively improved spot weldability and surface properties as steel sheet characteristics by controlling not only the process but also the composition and the temperature of the surface layer portion and the internal temperature of the continuously cast slab separately. With the goal.

[0008]

The above-mentioned problems can be solved by the following means. % By weight, C: 0.005% or less, N: 0.004% or less, M
n: 0.03 to 2.7%, Si: 0.8% or less, P: 0.1% or less, S:
0.001-0.02%, sol.Al: 0.01-0.08%, Ti: 0.00
A steel sheet containing 5 to 0.12% and having the following relationships among the above components and having excellent spot weldability and surface properties. (N _e / 14) + (S / 32) ≧ 0.00023 (Mn + T _{i e} ) / S ≧ 8 where N _e = min ((14 Ti / 48), N) T _{i e} = Ti- (48 N _e / 14) .

In addition to the ingredients described in,
B: 0.0002 to 0.003% is contained, and the steel sheet excellent in spot weldability and surface properties described in [1].

In addition to the components described in or, Nb: 0.003 to 0.05% in weight% is contained, and the steel sheet excellent in spot weldability and surface properties described in or.

A method for producing a steel sheet having excellent spot weldability and surface properties, which comprises sequentially performing the following steps (a), (b), and (c). (A)% by weight, C: 0.005% or less, N: 0.004% or less, M
n: 0.03 to 2.7%, Si: 0.8% or less, P: 0.1% or less, S:
0.001-0.02%, sol.Al: 0.01-0.08%, Ti: 0.0
A step of continuously casting a slab containing 05 to 0.12% and having the following relationships among the above components, (N _e / 14) + (S / 32) ≧ 0.00023 (Mn + Ti _e ) / S ≧ 8 N _e = min ((14 Ti / 48), N) T _{i e} = Ti- (48 N _e / 14). (B) Without cooling the continuously cast slab to room temperature, once a minimum of 5 mm from the slab surface, up to 20% of the maximum slab thickness is once Ar ₁
After cooling to the following, sensible heat inside the slab, or by heat retention or heating for a short time, the surface of the slab again
A step of reheating to a range of 1000 to 1150 ° C., and (c) a step of immediately hot rolling the reheated slab.

In addition to the ingredients described in,
B: 0.0002 to 0.003% is contained, The manufacturing method of the steel plate excellent in the spot weldability and surface quality as described in the above.

The method for producing a steel sheet having excellent spot weldability and surface properties as described in or, further containing Nb: 0.003 to 0.05% in weight% in addition to the components described in or.

A method for producing a cold-rolled steel sheet having excellent spot weldability and surface properties, characterized by comprising the following steps. (A) to the step described in any one of, and (b)
A step of further cold rolling the hot rolled steel sheet obtained in the above step.

[0015]

In the direct rolling process of the present invention, only the surface layer portion of the continuously cast slab, mainly the slab, is forcibly cooled and the austenite → ferrite and ferrite → austenite transformations are repeated mainly by recuperation to achieve a finer structure. The hot-rolling cracks caused by the Fe-S-based compound are prevented by making the Fe-S-based compound deposited on the pro-eutectoid austenite grain boundary and the austenite grain boundary after repeated transformation disagree. In addition, inside, as with normal direct rolling,
In order to be hot-rolled without going through the cooling-reheating process,
Not only TiN and MnS but also TiS precipitates non-equilibrium and is finely dispersed.

Such finely dispersed precipitates have an effect of suppressing the crystal grain coarsening in the spot welding heat affected zone. Therefore, in the method according to the present invention, not only energy saving and process saving can be achieved but also surface properties and spot weldability are greatly improved.

The reasons for limiting the component composition of the present invention will be described below. When C is present in a solid solution state, it not only hardens the steel but also causes deterioration at room temperature aging, which adversely affects the formation of a texture in cold rolling-annealing material, which is advantageous for improving deep drawability. It is desirable that the content be 0.005% or less.

Since N has the same adverse effect as C when it exists in a solid solution state, it is desirable to reduce it as much as possible, and its upper limit is made 0.004%. However, when fine TiN is present, grain coarsening of the spot-welded heat-affected zone is suppressed, so it is necessary to secure the amount described below in relation to Ti and S.

S reduces the hot rolling property and causes surface cracks during hot rolling, so the upper limit is made 0.02%. However, if it is too small, the peelability of the scale deteriorates, and at the same time, it becomes impossible to secure the fine MnS and TiS necessary for suppressing the coarsening of the crystal grains in the heat affected zone of spot welding.
1% or more is necessary. Therefore, the content is 0.001-0.
02%

Mn has an effect of suppressing hot brittleness due to S, and at the same time, fine MnS can prevent coarsening of crystal grains in the heat-affected zone of spot welding. For the S amount range of the present invention, at least 0.03% must be added. However, addition of a large amount deteriorates formability such as deep drawability even with IF steel, so the upper limit is made 2.7%.

Si is an element that contributes to an increase in strength in spite of its small adverse effect on formability, but addition of a large amount causes remarkable deterioration of formability, so the upper limit is made 0.8%.

Similar to Si, P is an element that contributes to the increase in strength in spite of its small adverse effect on formability, but addition of a large amount causes embrittlement due to segregation, so the upper limit is made 0.1%.

It is necessary to add sol.Al as Al for deoxidizing and fixing solid solution N. If it is less than 0.01%, the effect cannot be obtained, and conversely, if it is more than 0.08%, it causes deterioration of formability such as deep drawability and at the same time causes a loss of economy, so the addition amount is set to 0.01 to 0.08%.

Ti forms TiN and further fixes S as TiS to suppress hot embrittlement similar to Mn to improve surface properties and at the same time, finely precipitates the crystal grains in the spot welding heat affected zone. Coarsening can be prevented.
However, excessive addition not only saturates the effect, but also causes cost increase, so the addition amount is 0.005-0.12%.
And

In the present invention, the above composition is used as a basic component, but if necessary, one or more of the following elements may be added.

Since Nb and B each have the effect of preventing secondary processing embrittlement, either one or both may be added. If it is less than 0.003% or 0.0002%, the effect is small, and if it exceeds 0.05% or 0.003%, the effect is not only saturated, but also the cost is increased.
Therefore, the addition amount of Nb and B is 0.003 to 0.05, respectively.
%, 0.0002 to 0.003%.

In addition, if V, Cr, Cu, Ni, etc. are each 1% or less and Sn, which is an impurity, is 0.02% or less, the effect of the present invention is not impaired. Absent.

Next, the manufacturing conditions and the important relational expressions in the present invention, (N _e / 14) + (S / 32) ≧ 0.00023 and (Mn + Ti _e ) / S ≧ 8, will be described. However, N
_{e = min ((14Ti / 48} ), N), Ti e = Ti- (48
N _e / 14). In addition, min ((14Ti / 48), N)
Means the smaller of (14Ti / 48) and N.

In the manufacturing method of the present invention, after continuous casting, the slab is directly fed and rolled by utilizing the heat retained by the slab without cooling to room temperature. In the method of reheating a slab that has become a normal cold ingot, TiN, MnS, or TiS is coarsened due to Ostwald growth during heating, but in the direct-feed rolling method, the slab is in a state of slab as compared with reheating. The time of exposure to 1000 ° C or higher is extremely short, and the plate thickness decreases during rolling even at temperatures lower than 1000 ° C, resulting in a higher cooling rate than that of slabs.

Therefore, the nucleation of the precipitate is delayed, and the precipitates are generated all at once in a short time, so that they are finely dispersed, and the grain coarsening of the spot-welding heat-affected zone is suppressed. Here, Ti is first deposited as TiN, and then the remaining Ti is deposited as TiS while competing with MnS.
As a result of examining how much such three kinds of fine precipitates should be secured, (N _e / 14) + (S / 32)
It was found that ≧ 0.00023.

FIG. 4 is a graph showing the relationship between (N _e / 14) + (S / 32) and the spot-welded joint strength. N _e is Ti
It means the amount of N as N, and since all S is MnS or TiS in the claims of the present invention, (N _e / 1
4) + (S / 32) is the atomic weight ratio of TiN and Ti
It represents the total amount of S and MnS, and as shown in FIG. 4, if this value is controlled to be 0.00023 or more, coarsening of crystal grains in the spot-welded heat-affected zone can be suppressed and improvement in welding strength can be achieved.

On the other hand, the surface layer portion of the cast slab is not cooled below the A ₃ transformation point in ordinary direct rolling which is hot-rolled, so that the low melting point FeS or the precipitation of coarse austenite grain boundaries is obtained. High Fe content (Fe,
Mn) S causes intergranular cracking during hot rolling and deteriorates the surface properties of not only the hot rolled plate but also the cold rolled plate and the plated steel plate. Therefore, the current situation is that the simultaneous improvement of surface and internal properties cannot be established by the conventional techniques.

Therefore, in the present invention, only the surface layer is A
r ₁ transformation point (specifically, about 890-1335C ^0.5 (℃))
It is forcibly cooled to the temperature below, and the surface is reheated to 1000 ° C or more and 1150 ° C or less by sensible heat inside the slab. However, if the surface is not reheated to 1000 ° C. or higher, not only the finish temperature of hot rolling is lowered and the stripability is deteriorated, but also the variation in strip thickness is increased and the surface quality is degraded.

However, when the recuperation temperature exceeds 1150 ° C., mainly sulfides are solid-resolved and dynamically precipitate during the hot rolling, and surface cracks of the hot rolled steel sheet become apparent, and the above recuperation method is used. The effect of will be lost. Therefore, the proper recuperation temperature range is 1000 to 1150 ° C. The region which has been cooled and reheated below this temperature not only has a fine structure due to repeated austenite → ferrite and ferrite → austenite transformations, but also precipitates (Fe, Mn) S-based inclusions in the former austenite grain boundaries. Since the position is different from the grain boundary during hot rolling, the grain boundary strength is significantly increased,
The surface quality of the thin steel sheet product at each stage is improved.

However, if (Mn + Ti _e ) / S ≧ 8 is not satisfied, free S is segregated non-equilibrium to new fine austenite grain boundaries to lower the hot ductility, so that reheat and (Mn + Ti _e ) / A combination of S ≧ 8 is essential.
Figure 3 is a diagram showing the relationship (Mn + Ti _e) / S and the hot rolled sheet surface flaw generation rate. Here, Ti _e denotes the amount of Ti can be a remaining TiS became TiN, if 8 times the sum of Mn and Ti _e which can be a sulfide S, 3
As shown in, the occurrence of surface defects due to hot brittleness can be avoided.

Although Ti and Mn are not equivalent from the atomic weight ratio, Ti and Mn have the same effect as a result because Ti has a higher diffusion rate. Since the precipitates in the forced cooling section have nucleated in the cooling stage, they become coarser in the recuperating stage than in the central portion. This is desirable from the viewpoint of suppressing hot embrittlement, but it loses its effect on suppressing crystal grain coarsening in the spot welding heat affected zone.

Here, FIG. 1 shows Ar per side.₁less than
Shows the relationship between the depth of the cooled surface layer and the surface defect rate of hot-rolled sheet.
Fig. 2 shows Ar per side₁Surface layer cooled below
It is a diagram showing the relationship between the area range and spot welded joint strength.
It Ar from the viewpoint of surface texture ₁Is forced to
As shown in Fig. 1, the surface layer is wide when the slab is a slab.
A minimum of 5 mm from the surface is required on each side of the surface
On the contrary, from the viewpoint of spot weldability, Ar₁transformation
As shown in Fig. 2, the surface layer that is forcibly cooled below the point is wide.
The surface layer must be 20% or less of the maximum total thickness per side of the width surface.
The effect of coarse TiN, TiS and MnS on the
Disappear. In the case of square slab, a minimum of 5 from 4 surfaces
mm, 20% of maximum total thickness. So Ar₁Forced to
The surface layer to be cooled should be at least 5 mm or more per side,
And, it shall be 20% or less of the maximum total thickness.

The limitation of the forced cooling and recuperating temperature and the depth of the surface layer part as well as the composition of components as described above are the main feature of the present invention.

The reheated slab may be hot-rolled as it is, and the slab surface does not exceed 1000 ° C. and 1150 ° C. for reasons such as adjusting the rolling schedule and compensating for temperature decrease at the slab edge. You may heat / heat the whole or local area for a short time. The heating time is not particularly specified, but it is preferably within 15 min in order to prevent coarsening or re-dissolution of precipitates.

Therefore, in the present invention, in continuous casting, only the surface layer portion of the continuously cast slab is forcibly cooled to below the Ar ₁ transformation point and the sensible heat inside the slab is utilized to resurface the surface.
Reheat to 1000 ° C or more and 1150 ° C or less and perform hot rolling without heating or by heating / holding heat for a short time.
Such recuperation can be achieved by changing the secondary cooling pattern of the continuous casting slab from early strong cooling to late weak cooling. The extent to which the surface is forcibly cooled to below Ar ₁ can be understood from the heat / cooling calculation, and can also be confirmed from the change in the cross-sectional structure of the slab after reheat.

Although the process after hot rolling is not particularly specified, the effect of the present invention can be obtained in any of hot rolled steel sheet, cold rolled steel sheet and surface treated steel sheet. As a basic process, for example, in the case of a hot rolled steel sheet, there is hot rolling-winding treatment-scale removal, and in the case of a cold rolled steel sheet, there is further cold rolling-continuous annealing or batch annealing-temper rolling. In the case of surface-treated steel sheets, after the scale of the hot-rolled sheet is removed, after cold rolling or after annealing, a method such as temper rolling, one-step or multi-step melting or electroplating and a combination thereof is generally used. Is a process. In addition, there is no problem even if processes such as leveling, intermediate annealing, grinding, and edge cutting are added.

[0042]

EXAMPLES Table 1 is a table showing the composition of the steel of the present invention, and Table 2
Is a table showing the composition of comparative steels. Further, Table 3 is a table showing the recuperative heat direct rolling conditions and characteristics of the steel of the present invention, and Table 4 is a table showing the reheat direct heat rolling conditions and characteristics of the comparative steel.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

Here, for some of the comparative steels, the slab was cooled to room temperature and reheated (steel 17-h), and a normal straight-rolled material in which the surface layer portion was not forcedly cooled to below the Ar ₁ transformation point ( Including steel 17-i). For hot rolling, slabs of 220 and 250 mm in thickness were finished at 880 to 910 ℃ to 2.4 to 4.5 mm, cooled on a runout table, and then wound at 600 to 680 ℃.

The cold-rolled steel sheet was further subjected to cold rolling after removing the oxide scale to 0.7 to 1.4 mm, continuous annealing at 800 to 880 ° C., and further 0.5% temper rolling after annealing. For steel 20-a, instead of continuous annealing, 750
Box annealing was performed at ℃. Hot-dip galvanized material (Steel 2-a) is cold-rolled, annealed at 820 ° C, and when it is cooled to 460 ° C, 60 g / m ² of hot-dip galvanizing is plated on one side, and then alloying is performed at 500 ° C. I went. Then, after 1.0% temper rolling, upper layer electroplating of 80% Fe-Zn alloy with 3 g / m ² per side was further applied. Organic coated steel sheet (Steel 8-
For a), after temper rolling, 88% Zn-Ni alloy electroplating 30 g / m ² , chromate layer 50 mg / m ² , resin layer 1 μm.
A m 2 composite coating was applied.

The tensile properties of such a thin steel sheet were evaluated by JIS No. 5 test pieces, and the spot weldability was
A tensile shear test was carried out in accordance with JIS Z3136. The tensile shear strength becomes higher as the plate thickness becomes thicker and as the base metal strength becomes higher. Therefore, in order to evaluate whether the obtained tensile shear strength is a sufficient value, the calculated value obtained by multiple regression of the conventional data {356.7 ・ t ^1.42・ (TS / 9.8)
^0.84 } and the measured value were compared. Therefore, if the tensile shear strength ratio, which is the ratio of the actually measured value and the calculated value, is close to 1, it was judged that the spot weldability was excellent. However, t: plate thickness (mm), TS: tensile strength (MPa).

The spot welding conditions are as follows. Electrode: CR type (Original diameter: 16 mm, Vertical angle: 120 °) Electrode tip diameter: 6 ・ t ^0.5 mm Electrode pressure: 3330 ・ tN Energization time: 12 ・ t cycle / 60Hz Holding time: 60 cycles / 60Hz Welding current: Chile Limiting current The surface texture was evaluated by the number of flaws of 5 mm or more per 1 m ² of hot-rolled steel sheet (viewing area 50 to 60 m ² ).

All of the invention steels are excellent in spot weldability and surface properties, whereas steels 6-d, 17-d, 19-d, which are forcibly cooled to Ar ₁ or less and have a shallow surface layer, are usually straight rolled materials. Steel 17-i, reheat temperature is too low 17-f, reheat temperature is too high 17-g, and (Mn + Ti _e ) / S ≧ 8
21-a, 22-a, 23-a, 24-a, which do not satisfy
In each case, many surface defects occur. On the contrary, steels 6-e, 17-e, 19 with deep surface layer that are forcibly cooled to Ar ₁ or less
-E, and cooled to room temperature-reheated steel 17-h, (N
Steel that does not satisfy _e / 14) + (S / 32) ≧ 0.00023 25
-A, 26-a, 27-a and 28-a are inferior in spot weldability.

[0052]

The steel sheet and the method for producing the same according to the present invention are steel sheets suitable for applications requiring good spot weldability such as automobiles, that is, hot-rolled sheets, surface-treated steel sheets, cold-rolled steel sheets, And a surface-treated steel sheet thereof, which is an invention having a great industrial utility value.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the depth of the surface layer portion cooled to Ar ₁ or less per one side and the occurrence rate of surface defects on hot-rolled sheet.

FIG. 2 is a diagram showing a relationship between a surface layer range cooled to Ar ₁ or less per one side and spot welded joint strength.

FIG. 3 is a diagram showing the relationship between (Mn + Ti _e ) / S and the occurrence rate of surface defects on hot-rolled sheet.

FIG. 4 is a diagram showing a relationship between (N _e / 14) + (S / 32) and spot-welded joint strength.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Keiji Yoshioka, Marunouchi 1-2-2, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Issei Okimoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside the Steel Pipe Corporation (72) Inventor Tomoya Izushi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Steel Pipe Corporation

Claims (7)

[Claims] 1. C: 0.005% or less, N: 0.004% by weight
Below, Mn: 0.03 to 2.7%, Si: 0.8% or less, P: 0.1% or less, S: 0.001 to 0.02%, sol.Al: 0.01 to 0.08%, T
i: A steel sheet containing 0.005 to 0.12% and having the following relationships among the above components and having excellent spot weldability and surface properties. (N _e / 14) + (S / 32) ≧ 0.00023 (Mn + T _{i e} ) / S ≧ 8 where N _e = min ((14 Ti / 48), N) T _{i e} = Ti- (48 N _e / 14) . 2. In addition to the components of claim 1, weight%
B: 0.0002 to 0.003% is contained, and the steel plate excellent in spot weldability and surface properties according to claim 1. 3. The spot weldability according to claim 1 or 2, characterized in that, in addition to the components according to claim 1 or claim 2, it contains Nb in an amount of 0.003 to 0.05% by weight. And a steel plate with excellent surface properties. 4. A method for producing a steel sheet having excellent spot weldability and surface properties, which comprises sequentially performing the following steps (a), (b) and (c). (A)% by weight, C: 0.005% or less, N: 0.004% or less, M
n: 0.03 to 2.7%, Si: 0.8% or less, P: 0.1% or less, S:
0.001-0.02%, sol.Al: 0.01-0.08%, Ti: 0.0
A step of continuously casting a slab containing 05 to 0.12% and having the following relationships among the above components, (N _e / 14) + (S / 32) ≧ 0.00023 (Mn + Ti _e ) / S ≧ 8 N _e = min ((14 Ti / 48), N) T _{i e} = Ti- (48 N _e / 14). (B) Without cooling the continuously cast slab to room temperature, once cool the surface of the slab to a minimum of 5 mm and up to 20% of the maximum slab thickness to Ar ₁ or less, and then sensible heat inside the slab. Or by heat retention or heating for a short time,
A step of reheating the surface of the slab again to a range of 1000 to 1150 ° C., and (c) a step of immediately hot rolling the reheated slab. 5. In addition to the components according to claim 4, weight%
B: 0.0002-0.003% is contained, The manufacturing method of the steel plate excellent in the spot weldability and surface quality of Claim 4 characterized by the above-mentioned. 6. The spot weldability according to claim 4 or 5, characterized in that, in addition to the components according to claim 4 or 5, it contains Nb: 0.003 to 0.05% by weight. And a method for manufacturing a steel sheet having excellent surface properties. 7. A method for producing a cold-rolled steel sheet having excellent spot weldability and surface properties, comprising the following steps. (A) a step according to any one of claims 4 to 6,
(B) A step of further cold rolling the hot-rolled steel sheet obtained in the above step.