JPH09310149A - Cold rolled steel sheet excellent in deep drawability, spot weldability and punchability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a deep drawability, punchability and spot weldability of a cold rolled steel sheet without checking its formability and mechanical properties by allowing specified amounts of C, Si, Mn, P, S, Al, N and Fe to be contained in a steel and regulating the size and the number of TiC base precipitates present in the steel. SOLUTION: This cold rolled steel sheet is the one having a compsn. contg., by mass, 0.001 to 0.01% C, <=0.1% Si, 0.1 to 0.3% Mn, <=0.05% P, 0.002 to 0.025% S, <=0.1% Al, <=0.005% N, and the balance substantial Fe. Furthermore, this compsn. satisfies the formula and inequality. In the formula, [SRT] denotes the heating temp. before hot rolling, and [element] denotes the mass % of each element. The number of TiC base precipitates with the size of <=50nm in the cold rolled steel sheet is suppressed to 5×10<5> pieces/mm<2> or below. For this suppression, the slab is heated at 1100 to 1200 deg.C, hot rolling is finished at (the Ar3 transformation point + 80 deg.C) to (the Ar3 transformation point -40 deg.C), and cold rolling and annealing are executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet excellent in deep drawing, spot welding, punching and the like and a method for manufacturing the cold-rolled steel sheet. It can be effectively used as a material for automobiles and automobiles.

[0002]

2. Description of the Related Art Cold-rolled steel sheets are widely used in automobiles, home appliances and the like. In addition to deep drawing workability, surface cleanliness, strength, dent resistance, weldability, punching are also used according to their respective applications. Properties such as corrosion resistance, paintability, and corrosion resistance are required. Particularly in recent steel sheets for automobiles and home appliances, the shape of parts has become more and more complicated in response to user's demands such as omission of pressing process and weight reduction. Various measures are taken to satisfy various characteristics. Among these, for deep drawability, improving the r value,
Alternatively, measures for reducing the yield stress and increasing the work hardening rate are particularly important from the viewpoint of maintaining the shape.

Specifically, a so-called IF in which carbon nitride is fixed by adding carbonitride forming elements such as Ti and Nb to ultra low carbon steel.
The use of thin steel sheets is increasing, and various reports have been made (Japanese Patent Publication No. 44-18066 and Japanese Patent Publication No. 54-1).
245, JP-A-59-67319, etc.). In general, IF thin steel sheets have low elongation and yield stress, and cold rolled steel sheets have extremely high r-values, and are excellent in various workability such as deep drawing and stretch flange forming. This is because it has extremely excellent suitability. However, while the IF-based thin steel sheet is excellent in workability, since it is an extremely low carbon steel, the crystal grains in the spot-welded heat-affected zone are likely to coarsen and the strength of the spot-welded joint is higher than that of the low-carbon Al-killed cold-rolled steel sheet. However, its application range was limited.

Therefore, the contents of Ti, Nb, and B are adjusted for the purpose of improving the joint strength in the spot-welded heat-affected zone (Japanese Patent Laid-Open Nos. 63-317625 and 63-317648, JP-A-63-317649
JP-A-63-317647 and JP-A-7-138.
699 gazette) etc. are proposed. Of these, the latter method is intended to achieve the intended purpose by forming fine Ti-based precipitates.
A large number of fine Ti-based precipitates of m (50 nm) or less are dispersed (Japanese Patent Laid-Open No. 63-317647), or
A method in which TiN, MnS, TiS, etc. are precipitated in a non-equilibrium state and finely dispersed by adopting a direct rolling process instead of a normal cooling-reheating process (Japanese Patent Laid-Open No. 7-13
No. 8699) is disclosed. However, when such fine precipitates are generated, the spot weldability is improved, but the processing characteristics such as deep drawability are remarkably deteriorated, and both the characteristics of the workability and the spot weldability cannot be provided. .

On the other hand, in the case of press-forming automobiles and home electric appliances, punching such as blanking is almost always performed before and after the process. There is a problem that the burr height of the end surface after punching is large as compared with the carbon Al killed cold rolled steel sheet. That is, when the steel sheet is used for the chassis of home electric appliances, computer cases, etc., it is often applied to the sliding part after punching, so the burr on the end face after punching should be a certain height or more. However, the conventional IF-based thin steel sheet is
This required characteristic was not sufficiently satisfied. On the other hand, low carbon Al killed steel sheets are often used because of their excellent punching workability, but it is considered that their action is due to the formation of cementite and MnS precipitates. That is,
C in steel coarsely precipitates as brittle cementite in grain boundaries and crystal grains, and this cementite serves as a crack initiation point during punching, which reduces the amount of burrs, and also causes die wear and shear load. Also becomes smaller. Furthermore, it is known that MnS precipitates act similarly to cementite. However, these cementite and MnS precipitates are also metallurgical factors that control the ultimate deformability of steel, and their existence as crack initiation points deteriorates the press workability due to the ultimate deformability at the same time. Is accompanied by. Therefore, there is a problem that the low carbon Al killed steel sheet cannot be applied to difficult-to-machine parts.

Therefore, while maintaining the excellent formability and mechanical properties of the conventional IF-type cold-rolled steel sheet, the burr height of the end face can be suppressed during punching such as blanking.
Moreover, in spot welding, it is strongly desired to develop a cold-rolled steel sheet suitable for automobile parts and home electric appliances which has good joint strength in the heat-affected zone.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is to provide excellent formability and mechanical properties of a conventional IF cold-rolled steel sheet. It is intended to establish a cold-rolled steel sheet which is excellent in spot weldability and punching workability while being secured, and which can be effectively utilized as a material for home electric appliances, automobile parts and the like, and a manufacturing method thereof.

[0008]

The constitution of the cold-rolled steel sheet according to the present invention which has been able to achieve the above-mentioned object is C in mass%:
0.001-0.01%, Si: 0.1% or less (not including 0%), Mn: 0.1-0.3%, P: 0.05%
Below (not including 0%), S: 0.002 to 0.025
%, Al: 0.1% or less (not including 0%) is satisfied, and N: 0.005% or less is suppressed, and [S] / [C] = 0.01 × {[SRT] −900 } ~ 0.04 x {[SRT] -900} ... (1) (where [element] represents the mass% of each element, and [SRT] represents the heating temperature (° C) before hot rolling) and (48 / 12) C + (48/32) S + (48/14) N ≦ Ti ≦ 0.10% (2), and the balance: Fe and inevitable impurities, the size of TiC-based precipitates present in the steel Is 50
The gist is that the number of nm or less is suppressed to 5 × 10 ⁵ pieces / mm ² or less.

In the cold rolled steel sheet, Nb: 0.03% or less (not including 0%), and / or
Or B: 0.005% or less (not including 0%) and TiC precipitates and Nb existing in the steel.
Those in which the size of both (C, N) -based precipitates is 50 nm or less in total is suppressed to 5 × 10 ⁵ pieces / mm ² or less, and the spot weldability and the like can be further improved. It is a mode.

The manufacturing method according to the present invention comprises a slab 1100 made of steel which satisfies the above-mentioned compositional requirements.
After heating in the temperature range of ˜1200 ° C., the hot rolling is finished in the temperature range of (Ar ₃ transformation temperature + 80 ° C.) to (Ar ₃ transformation temperature −40 ° C.), and then wound, and then cold rolling is performed. TiC present in the steel by annealing after
For controlling the size of the system precipitates of 50 nm or less, or the size of both the precipitates and the Nb (C, N) system precipitates of 50 nm or less, to a total of 5 × 10 ⁵ pieces / mm ² or less. There is a gist.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the IF type steel sheet is
Despite having excellent formability and mechanical properties, it was inferior to the low carbon Al-killed cold rolled steel sheet in terms of punching workability and spot weldability. Various studies have been conducted to provide these characteristics, but if the punching workability and spot weldability are improved, there is a problem that the mechanical properties inherent to the IF-based steel sheet, particularly the deep drawing, decrease. .

[0012] Therefore, the present inventors provide a cold-rolled steel sheet capable of significantly improving punching workability and burring workability while securing excellent press formability and the like in an ultra-low carbon IF type steel plate. As a result of diligent studies, the composition of the steel material was specified, and in particular Ti ₄ C ₂ S
_It has been already reported and found that the intended purpose can be achieved by allowing a specific amount of a precipitate of ₂ and TiS of a specific size to be present (Japanese Patent Application No. 7-201197).
Issue). However, since the publication does not consider the improvement of spot weldability at all, further study is required to provide this property. In the present invention, in addition to the characteristics of formability and punching workability that have been the object of improvement in the above publication, as a result of further studying the above technical contents so as to have spot weldability, all of these characteristics are obtained. The present inventors have completed the present invention by finding out what kind of Ti-based precipitate is effective for each characteristic and how to control the distribution state thereof. Specifically, the present invention has the following skeleton.

Punchability In the above publication, both punching workability and burring workability are described.
To make them compatible, Ti _Four C_Two S_Two And the size of TiS
While controlling in the range of 100 to 1000 nm, the precipitation
1 x 10 dispersion amount^Four ~ 5 × 10^Five Pieces / mm^Two Range
Was in control. That is, these Ti-based compounds are relatively
Due to the large precipitates, the steel sheet was subjected to strong shear deformation.
Shape of voids and cracks near the precipitate
Burrs that occur on the punched end face due to the acceleration of the formation and its progress.
Is thought to be kept as small as possible. The present invention
Then, in addition to providing the above characteristics, spot welding
Dedicated to providing ultra low carbon IF cold rolled steel sheet
As a result of examination, Ti, which is the most effective in improving these properties,
Ti-based compound_Four C_Two S_Two I found that
You.

Spot Weldability As described above, the decrease in spot weldability is due to the coarsening of the crystal grains in the heat affected zone of the spot weld. A more detailed study of this phenomenon revealed that the steel sheet was not welded during spot welding. It was found that some of the material melted and the temperature around it became extremely high. That is, in the conventional ultra-low carbon IF steel sheet, there is nothing that prevents grain boundary migration, and there is nothing that can act as a nucleation site when transforming from an austenite phase to a ferrite phase in the cooling process. Therefore, the tendency of coarsening becomes remarkable. Therefore, it prevents grain boundary migration and also acts as a nucleation site during the transformation from the austenite phase to the ferrite phase (thus, it must be a stable precipitate in the low austenite temperature range). It was found that the microstructure of the spot-welded heat-affected zone can be remarkably improved by the presence of a precipitate, specifically, a precipitate having a size of 100 nm or more. Furthermore, as such a precipitate,
It has also been found that Ti ₄ C ₂ S ₂ is optimal rather than TiS.

Deep drawability According to the findings of the present inventors, it is difficult to obtain good deep drawability even with an extremely low carbon IF steel sheet when a large amount of fine precipitates of 50 nm or less are present. It turned out that As described above, among the conventional techniques, a steel sheet (Japanese Patent Laid-Open No. 63-317647) in which a large number of fine Ti-based precipitates of 50 nm or less are dispersed is reported for the purpose of providing an IF-based steel sheet having excellent spot weldability. However, according to the above-mentioned findings, in such a steel sheet, the spot weldability may certainly be improved, but the excellent deep drawability originally possessed by the IF-based steel sheet is significantly lowered. Therefore, in order to secure the deep drawability as well, it is necessary to reduce the fine precipitates of 50 nm or less as much as possible.

As described above, in the present invention, (a) the size is 10 for the purpose of improving spot weldability and punching workability.
In addition to the presence of large precipitates of 0 nm or more, (b) I
For the purpose of ensuring the deep drawability originally possessed by F-based steel sheets, 5
It has been found that by reducing fine precipitates of 0 nm or less as much as possible, a cold rolled steel sheet excellent in all properties can be obtained without impairing any of the properties of deep drawing property, spot weldability and punching property. The invention was completed.

Hereinafter, various precipitates existing in the ultra low carbon IF type cold rolled steel sheet [TiC, Ti ₄ C ₂ S in Ti added steel]
₂ , TiS, TiN, FeTiP, Ti ₂ O, etc .; Nb
With regard to Nb (C, N) etc. in the added steel], the results of examining their influence on the formability, punching workability and spot weldability of the cold rolled steel sheet are described below.

Ti ₄ C ₂ S ₂ : Ti ₄ C ₂ S _{2 has} a large size of 100 to 1000 nm, and the above (a) and (b)
The compound is essential for achieving the object of the present invention in that it is a compound that simultaneously satisfies the above requirement. That is, since the size satisfies the requirement of (b) above, it does not adversely affect the moldability, and since it also satisfies the requirement of (a) above, punching property is improved by making it exist in a certain amount or more. Not only that, it can effectively act as a nucleation site when transforming from an austenite phase to a ferrite phase in the cooling process during spot welding.

However, if the above-mentioned precipitate is to be produced, and if C is contained in the steel in an amount of 0.001% or more as in the present invention, under normal manufacturing conditions, the cooling and winding processes after hot rolling are performed. TiC and Nb (C, N) are generated, and the desired T
i ₄ C ₂ S ₂ cannot be obtained. As will be described later, TiC and Nb
The size of (C, N) is too small and exerts an adverse effect on the achievement of the object of the present invention. Therefore, Ti that contributes to the improvement of spot weldability and punchability while suppressing the formation of these compounds. It is necessary to promote the production of ₄ C ₂ S ₂ .

Therefore, in the present invention, a predetermined amount of Ti ₄ C ₂ S is used.
_{In order} to obtain ₂ , S corresponding to the amount of C is added, that is, S /
It was decided to specify by C ratio. The production rate of Ti ₄ C ₂ S ₂ depends on the heating temperature before hot rolling (may be abbreviated as [SRT]). Therefore, in the present invention, Ti ₄ C ₂ S _{2 is produced.}
The range of the S / C ratio, which is an index for the generation of, was defined by the following formula (1). S / C = 0.01 × {[SRT] −900} to 0.04 × {[SRT] −900} ... (1) ([SRT] represents the heating temperature (° C) before hot rolling)

[SRT] is 1050 to 1250 ° C.
It is preferred that If it is less than 1050 ° C, it is difficult to finish the hot rolling at (Ar ₃ transformation temperature-40 ° C), while if it exceeds 1250 ° C, even if the S / C ratio is increased, T
It is difficult to generate i ₄ C ₂ S ₂ . More preferably, it is 1100 to 1200 ° C.

Further, since Ti ₄ C ₂ S ₂ also depends on the heating temperature and finishing temperature in hot rolling, it is necessary to consider these requirements in the manufacturing process (described later).

TiC-based and Nb (C, N) -based : The size of these precipitates is very small, about 10 nm.
Therefore, the above condition (b) is not satisfied, not only the deep drawability is significantly deteriorated, but also the punching property improving effect is very small. Also regarding spot weldability, since it melts in the austenite temperature region associated with heating, it cannot act as a nucleation site during transformation into a ferrite phase. Taking these points into consideration, it is preferable to suppress the formation of the above-mentioned precipitates as much as possible, and substantially 5 × 10 ^{5 in} total.
It is necessary to control the number of pieces / mm ² or less.

FeTiP : The size of the precipitate is Ti ₄
It is equivalent to C ₂ S ₂ and contributes to the improvement of punchability, but it significantly deteriorates the deep drawability and decomposes in the austenite temperature range, so it acts to prevent coarsening of crystal grains in the heat affected zone of spot welding. Can't get Therefore, in order to achieve the object of the present invention, it is preferable that the above-mentioned precipitate is not formed as much as possible.

In the above-mentioned publication, TiS : TiS is positively present within a predetermined range together with Ti ₄ C ₂ S ₂ as a precipitate that contributes to improvement of punching workability and does not hinder burring work and the like. It is a thing. However,
It was found that the precipitate does not act effectively in achieving the object of the present invention. That is, there are two types of TiS, one of which is extremely fine, which is generated in the ferrite region, and the other of which is relatively large (which is slightly larger than the size of Ti ₄ C ₂ S ₂ ), which is generated in the austenite region. Since it does not meet the requirement of 1), it not only deteriorates the deep drawability but also does not contribute to the improvement of spot weldability and punchability. On the other hand, the latter has a feeling that an effect of improving the punching property is obtained in terms of size, but in reality, it is very soft and therefore hardly contributes to the improvement. Further, when TiS is generated, C in the steel is present as fine TiC, which is not preferable for achieving the object of the present invention, and when it is added in excess, it causes a surface defect.

TiN : Its effect is almost the same as that of TiS produced in the austenite region described above, and it hardly contributes to the achievement of the object of the present invention. Further, it is extremely difficult to control the amount of N in practical steel, and therefore it is not easy to utilize the Ti compound for improving punching workability and the like.

As described above, according to the present invention, the extremely low carbon Ti-containing I
Among various compounds formed in F-based steel sheet, Ti is particularly
_{In order} to accelerate the formation of ₄ C ₂ S ₂ , S at the heating temperature before hot rolling
The intended purpose could be achieved by defining the / C ratio and suppressing the formation of fine precipitates (TiC, etc.) that hinder the formability such as deep drawability. Next, the reason for defining the composition of the steel material in the present invention will be clarified.

C: 0.001 to 0.01% C is an element indispensable for improving spot weldability and punchability by forming Ti ₄ C ₂ S ₂ by combining with Ti as described later. , At least 0.001% must be contained. However, if the amount of C is too large, the improvement effect is saturated, the amount of TiC produced is increased, and the mechanical properties such as r value, elongation, and yield stress are reduced.

Si: 0.1% or less (not including 0%) Si is an element that enhances the strength without lowering the ductility, but if it is contained in a large amount, an oxide film is generated during annealing, and then electricity is applied. During plating, the plating thickness becomes non-uniform, resulting in uneven plating and adversely affecting the chemical conversion processability. Particularly, when the cold-rolled steel sheet is alloyed and hot-dip galvanized, an unplated portion is likely to be formed due to the formation of the oxide film, so that the Si amount needs to be suppressed to 0.1% or less.

Mn: 0.1-0.3% If the content of Mn is less than 0.1%, not only FeS is generated to cause hot cracking, but also a huge cost is required for its reduction. It is uneconomical. On the other hand, when the addition amount is large, MnS that contributes to the improvement of spot weldability and punchability
However, the deep drawability is deteriorated, so the upper limit is made 0.3%.

P: 0.05% or less (not including 0%) P may form a solid solution in steel or may form FeTiP-based precipitates. In the case of the former, although it acts as a strengthening element, it adversely affects the spot weldability and punchability. On the other hand, when it comes to the form of precipitates such as the latter, it has some improvement effect on punching, but on the other hand, it has a spot weldability. With respect to (1), since it melts in the austenite temperature range, it cannot act effectively as a nucleation site during transformation into a ferrite phase, and further deep drawability is greatly reduced. Therefore, taking these factors into consideration, the upper limit of P is set to 0.
05%.

S: 0.002-0.025% S is a main precipitate of the present invention, together with Ti described later, and T.
It is an extremely important element for the production of i ₄ C ₂ S ₂ . If the amount of S added is less than 0.002%, desired precipitates are hardly obtained, and further, good spot weldability and punchability cannot be exhibited. However, if it is added excessively, the occurrence rate of surface defects increases, and fine TiS precipitates in the ferrite phase or promotes the formation of TiC. It is necessary that the morphology of the precipitates is mainly Ti ₄ C ₂ S ₂ by adding an appropriate amount of S. From such a viewpoint, the upper limit of S is set to 0.
To 025%.

Al: 0.1% or less (not including 0%) Al acts as a deoxidizing agent or the like. However, even if added in a large amount, it is uneconomical since only Al ₂ O ₃ which hardly contributes to the improvement of spot weldability and punching workability is produced, so it is desirable to suppress it to 0.1% or less.

N: 0.005% or less (including 0%) N combines with Ti to form a TiN precipitate.
Has almost no effect of improving spot weldability and punchability,
If added in a large amount, the deep drawability will deteriorate, so the upper limit is made 0.005% or less. Ti: The Ti amount needs to satisfy the following expression (2). (48/12) C + (48/32) S + (48/14) N ≦ Ti ≦ 0.10% (2)

That is, Ti is Ti which is the main precipitate of the present invention.
_Although it is an indispensable element for the production of ₄ C ₂ S ₂ , it also serves as an element for producing TiN, which adversely affects the present invention. Therefore, the N content in the above range is completely fixed, and the formability and non-aging resistance are improved. In order to give it, the upper limit needs to satisfy the left side of Formula (2). On the other hand, the above addition action of Ti is saturated when it exceeds 0.10%, so this is made the upper limit.

In the present invention, the contents of the above-mentioned elements are basically defined, but desired properties can be obtained by controlling the Nb and / or B-added steel within the following range. .

Nb: 0.03% or less and / or B: 0.
005% or less (all elements do not contain 0%) Of these, Nb forms extremely fine Nb (C, N), which deteriorates deep drawability. Therefore, in the present invention, such a precipitate is formed. It is necessary to exist in a solid solution state without being generated as much as possible, whereby the spot weldability can be improved. From such a viewpoint, the lower limit is 0.005%.
The above content is preferable, but if it exceeds 0.03%, not only the effect is saturated but also Nb (C, N) is formed, so the upper limit is preferably made 0.03% or less.

In the Ti-containing steel of the present invention, B is N
Since it is fixed as iN, it exists in a solid solution state without producing a precipitate such as BN. Therefore, B itself has almost no effect of suppressing the burr height generated during punching,
From this point of view, it is not an essential component, but if it is contained in an amount of 0.0002% or more, spot weldability becomes good. However, the effect is saturated at about 0.005%, and beyond that, the deep drawability deteriorates rather.

As described above, the cold rolled steel sheet of the present invention is
The most important Ti to achieve the purpose _Four C_Two S_Two Deposit
In order to form mainly, the S / C ratio is specified in the steel composition.
It is necessary to determine
It is also important to control the hot rolling temperature and winding conditions.
It will be important. Specifically, in order to achieve the object of the present invention
Is a slab made of steel that satisfies the above composition requirements.
After heating in the temperature range of 1100 to 1200 ° C., (Ar
_Three Transformation temperature + 80 ° C) ~ (Ar_Three Transformation temperature-40 ° C)
After hot rolling within the temperature range, wind up and then cool.
It was confirmed that hot rolling and annealing should be performed. In addition,
If it is manufactured under the conditions of
C-based precipitates with a size of 50 nm or less, or
The size of both the source and the Nb (C, N) -based precipitate is 50 nm.
5 x 10 in total^Five Pieces / mm^Two With suppression below
I also knew that I could do it.

If the heating temperature is less than 1100 ° C., Mn
When S precipitates and the temperature becomes higher than 1200 ° C, the precipitate becomes TiS. In any case, an appropriate amount of T
i ₄ C ₂ S ₂ cannot be obtained. Further, although the winding temperature after hot rolling is not particularly specified, when the temperature exceeds 700 ° C., Ti ₄ C ₂ S ₂ system precipitates become FeTi.
At the low temperature of less than 600 ° C., TiC is formed at most, and the desired Ti
₄ C ₂ S ₂ system cannot be obtained. Therefore, the winding temperature is preferably 600 to 700 ° C.

The finishing temperature of the hot rolling should be set in the range of (Ar ₃ transformation temperature + 80 ° C) to (Ar ₃ transformation temperature -40 ° C). If the hot finishing temperature is too low, FeTiP is formed, and the punching workability deteriorates due to the presence of itself and solid solution C. Conversely, if it is too high, M
This is because the amount of nS produced increases and the desired Ti ₄ C ₂ S ₂ -based precipitate cannot be obtained.

The cold-rolled steel sheet of the present invention thus obtained can of course be commercialized as it is, but for the purpose of rust prevention, electroplating, hot dip galvanizing, phosphate treatment, chromate treatment, etc. Chemical treatment,
Further, it is of course effective to apply a surface treatment such as an organic resin coating treatment.

Hereinafter, the constitution and operational effects of the present invention will be specifically described with reference to examples. However, the present invention is not limited by the following examples, and is within a range applicable to the gist of the preceding and the following. It is also possible to make appropriate modifications and implement them, and all of them are included in the technical scope of the present invention.

[0044]

【Example】

Example 1 After heating steel materials having the components shown in Tables 1 and 2 to 1160 ° C.,
Hot rolling was performed under the conditions of a hot rolling finish temperature of about 920 ° C and a winding temperature of about 680 ° C to obtain a hot rolled steel sheet having a plate thickness of 3.2 mm ^t . Then, after cold rolling to 0.8 mm ^t , continuous annealing was performed at 830 ° C. for 1 minute and then 0.5% temper rolling was performed. For each cold-rolled steel sheet thus obtained, the number of precipitates of 50 nm or less was measured, and the following characteristics were evaluated.

[Tensile Properties] Deep drawability was evaluated by measuring tensile properties such as r value. Among them, the r value was measured by applying a tensile strain of 15% using a JIS No. 13B tensile test piece and measuring other characteristics (Y
S, TS and El) were evaluated with JIS No. 5 tensile test pieces.

[Spot Weldability] A tensile shear strength test was carried out in accordance with JIS Z3126. Specifically, the minimum welding proper current is 4.8 mmφ CF type electrode,
The welding time was evaluated under the conditions of 8 cycles and the applied pressure: 200 kgf, while the welding strength was evaluated under the condition that the welding current was 7.5 KA.

[Punching] Clearance: 8m, 30m
It was evaluated by measuring the burr height of the end face punched to mφ. Table 3 shows the obtained results. The [S] / [C] ratio (described as S / C in the table) defined by the formula (1) of the present invention is 2.6 to 10.4 in this embodiment.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

In the table, steel types B to D, H to K, and R to T
(Example in which Nb and / or B is added) is an example of the present invention,
It can be seen that the tensile properties, spot weldability, and punchability all have very good properties. On the other hand, other steels have the following problems.

Since the steel types A, F, G and L have a small S / C ratio and a large number of precipitates of 50 nm or less, the tensile properties, spot weldability and punchability are all inferior to those of the present invention. It is a thing. On the contrary, since the steel type E has a large S / C ratio, not only these characteristics are deteriorated, but also surface defects such as blisters and slivers frequently occur on the surface of the steel sheet after cold rolling.

Since the steel type M has a large amount of C, the S / C ratio is small, the amount of Ti is large, and the number of precipitates having a size of 50 nm or less is large. Steel type N
However, since the amount of Si is large, unevenness in plating occurs due to an oxide film formed during electrogalvanizing, and plating adhesion during hot dip galvanizing decreases. Further, since the steel type P has a large amount of P, the tensile properties and the secondary workability are inferior, and the steel type Q has a large amount of N, so that the tensile properties are deteriorated.
From the above results, it can be seen that those which do not satisfy the chemical composition and the number of precipitates specified in the present invention cannot achieve the intended purpose.

Example 2 Using steel types B and H in Table 1, hot rolling was performed under the conditions shown in Table 4 to produce a hot rolled steel sheet having a plate thickness of 3.2 mm ^t , and then a plate thickness of 0.8 mm ^t was obtained. Cold rolling was performed, and continuous annealing was performed at 830 ° C. for 1 minute, and then 0.5% temper rolling was performed. Various characteristics of each cold-rolled steel sheet thus obtained were evaluated in the same manner as in Example 1. The results are shown in Table 5.
It describes in. For steel type B, S based on the amount of elements
/ C ratio was calculated to be 3.3, finishing temperature was 854-974
℃ (Ar ₃ points = 894 ℃), S / C for steel type H
The ratio is 3.2, and the finishing temperature is 846 to 966 ° C. (Ar ₃ point =
886 ° C).

[0055]

[Table 4]

[0056]

[Table 5]

Those satisfying the hot rolling conditions of the present invention (B2 to
B4, B6; H2 to H4, H6) all have excellent properties. On the other hand, since the heating temperature is high, the S / C ratio specified in the present invention is exceeded, and the TiC-based precipitates often have a size of 50 nm or less. Steel types B1 and H1; the finishing temperature is high and the TiC-based precipitates are high. Steel types with a size of 50 nm or less are often B5, H5: Steel types with low finishing temperature B7, H7
Is inferior in any of tensile properties, spot weldability, and punchability, and it is understood that all the properties required by the present invention cannot be satisfied.

[0058]

The present invention is constructed as described above, and S
/ C ratio and the amount of Ti in the to control the steel Ti ₄ C ₂ S
By mainly forming the ₂ system precipitates, it is possible to provide a cold-rolled steel sheet excellent in punching workability and spot weldability without impairing formability and mechanical properties, and to the method of the present invention. According to this, the cold-rolled steel sheet having such characteristics can be industrially reliably manufactured.

Claims (4)

[Claims] 1. C: 0.001 to 0.01 in mass%
%, Si: 0.1% or less (not including 0%), Mn:
0.1 to 0.3%, P: 0.05% or less (not including 0%), S: 0.002 to 0.025%, Al: 0.
Satisfies the requirement of 1% or less (not including 0%), and N
: 0.005% or less and [S] / [C] = 0.01 × {[SRT] −900} to 0.04 × {[SRT] −900} (1) (However, each [element] is Mass% of elements, [SRT] represents heating temperature (° C) before hot rolling, and (48/12) C + (48/32) S + (48/14) N ≦ Ti ≦ 0.10% (2 ) And the balance: Fe and unavoidable impurities, and the size of TiC-based precipitates present in the steel of 50 nm or less is suppressed to 5 × 10 ⁵ pieces / mm ² or less. Cold rolled steel sheet with excellent deep drawability, spot weldability, and punchability. 2. In addition to containing Nb: 0.03% or less (not including 0%) and / or B: 0.005% or less (not including 0%) as another element, in the steel Of the TiC-based precipitates and Nb (C, N) -based precipitates present in 2
The number is suppressed to 10 ⁵ pieces / mm ² or less.
The cold rolled steel sheet according to. 3. After heating a slab made of steel satisfying the compositional requirements described in claim 1 in a temperature range of 1100 to 1200 ° C., (Ar ₃ transformation temperature + 80 ° C.) to
The size of the TiC-based precipitates present in the steel is 50 nm or less by finishing the hot rolling in the temperature range of (Ar ₃ transformation temperature −40 ° C.), winding it, and then performing cold rolling and annealing. The method for producing a cold-rolled steel sheet having excellent deep drawability, spot weldability, and punchability, which is characterized in that the content of the cold-rolled steel sheet is suppressed to 5 × 10 ⁵ pieces / mm ² or less. 4. After heating a slab made of steel satisfying the compositional requirements described in claim 2 in a temperature range of 1100 to 1200 ° C., (Ar ₃ transformation temperature + 80 ° C.) to
After the hot rolling is completed in the temperature range of (Ar ₃ transformation temperature −40 ° C.), the rolling is performed, and then cold rolling is performed to anneal the TiC-based precipitates and Nb existing in the steel.
Excellent in deep drawability, spot weldability, and punchability, which are characterized in that the total amount of (C, N) -based precipitates having a size of 50 nm or less is suppressed to 5 × 10 ⁵ pieces / mm ² or less. Manufacturing method of cold rolled steel sheet.