JPH10168544A - Cold rolled steel sheet excellent in blanking property and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a cold rolled steel sheet having a good blanking property by controlling the structure of a Ti-If steel into a microstructure improving its punchability and clarifying producing conditions for obtaining the structure. SOLUTION: This cold rolled steel sheet has a compsn. contg., by weight, <=0.003% C, <=0.3% Si, 0.05 to 0.5% Mn, <=0.03% P, 0.003 to 0.015% S, 0.03 to 0.06% Sol.Al, 0.02 to 0.08% Ti and <=0.003% N. In the steel, when the average ferrite grain sizes in the surface layer part within 15% of the sheet thickness from the surface of the steel sheet and in the center part of the sheet thickness other than the part within 15% of the sheet thickness from the surface of the steel sheet are respectively defined as ds and db, the ratio of ds/db is regulated to <=0.95.

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 having a particularly good punching property and a method for producing the same, particularly in a cold-rolled steel sheet in which a large number of holes are drilled such as various electric parts.

[0002]

2. Description of the Related Art IF steel (Interstitial) in which a carbonitride forming element such as Ti or Nb is added to an ultra-low carbon steel.
Free Steel) has excellent press formability and deep drawability, and is therefore often used as a material for automobiles and a material for electric parts. Particularly, in the electrical industry, a cold-rolled steel sheet used is often subjected to a large number of drillings for screw holes. At this time, punching is almost always performed by punching, and burrs generated by punching may lead to problems in conductivity in electrical equipment and problems in threading work.

Under such a background, as a method for improving the punchability of a cold-rolled steel sheet, Japanese Patent Application Laid-Open No.
Utilizing MnS in steel by increasing the amount
No. 73457, a method using TiS by increasing the amount of S, Japanese Patent Application Laid-Open No. 8-73692, a method using TiN generated in steel by increasing the amount of N, and a method of increasing the strength of a steel sheet. Is disclosed.

[0004]

However, Japanese Patent Application Laid-Open No.
-230748, JP-A-6-73457,
In each case, the techniques disclosed in Japanese Patent Application Laid-Open No. H8-73992 increase the amounts of S and N, and the formability of the steel sheet is not always good. As for measures for increasing the strength of a steel sheet, the shape after forming is important for a material for electric parts, and springback is a problem. On the other hand, for example, as disclosed in JP-A-3-27739,
By making the components of the surface layer and the central part of the cold-rolled steel sheet different components,
There is also a technology to improve the burr resistance by hardening only the surface layer, but it is not realistic in the current thin steel plate manufacturing technology to vary the steel composition at a thickness of about 0.7 to 1 mm. It is easily assumed that the cost will increase.

The object of the present invention has been made in view of such circumstances, and it is necessary to control the microstructure to improve the punchability of Ti-IF steel and to adjust the manufacturing conditions for obtaining the microstructure. It is an object of the present invention to provide a cold-rolled steel sheet having good punching properties and a method for manufacturing the same.

[0006]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The cold-rolled steel sheet of the present invention has a C: 0.003 by weight%.
% Or less, Si: 0.3% or less, and Mn: 0.05 to
0.5%, P: 0.03% or less, S: 0.003 to
0.015% and Sol. Al: 0.03 to 0.06%
And Ti: 0.02 to 0.08% and N: 0.003%
The average ferrite in the central portion of the sheet thickness excluding the surface layer portion having a thickness of 15% or less from the surface of the steel sheet and the thickness of 15% or less from the surface of the steel sheet (hereinafter, this portion is referred to as the central portion of the sheet thickness). When the particle diameters are ds and db, respectively, ds /
It is a cold rolled steel sheet having excellent punchability, wherein db is 0.95 or less. (2) The cold-rolled steel sheet of the present invention further comprises B:
The cold rolled steel sheet having excellent punchability as described in (1) above, which contains 0.0002 to 0.0015%.

(3) The blank according to the above (1) or (2), wherein the cold-rolled steel sheet of the present invention further contains 0.005 to 0.03% by weight of Nb. This is a cold rolled steel sheet with excellent heat resistance.

(4) The method for producing a cold-rolled steel sheet according to the present invention described in (1) above comprises the steps of: heating the continuously cast steel to 1000 to 1200 ° C .; and a step of performing a final rough rolling at a temperature range Ar ₃ point of the steel, the rough rolling steel Ar ₃ or more points (Ar ₃
+100) Reheating at a heating rate of 5 ° C / s or more to a temperature range of less than 0 ° C, performing finish rolling at ₃ points or more of Ar, winding, and cold rolling the hot-rolled steel sheet, and recrystallization temperature to 850
A step of annealing at ℃ and subjecting to temper rolling at a pressure regulation rate of 1.5% or less. (5) The method for producing a cold-rolled steel sheet according to the present invention, wherein the steel sheet further contains B: 0.0002 to 0.0015% by weight. This is a method for producing a cold-rolled steel sheet having excellent punchability.

(6) In the method for producing a cold-rolled steel sheet according to the present invention, the steel sheet may further comprise Nb: 0.005 to 0.03 by weight%.
%, Wherein the cold-rolled steel sheet is excellent in punchability as described in the above item (4) or (5).

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor controls the microstructure to improve the punching property of Ti-IF steel and clarifies the manufacturing conditions for obtaining the structure, thereby improving the punching property. In order to obtain a cold-rolled steel sheet, intensive studies were conducted on the steel composition and the metal structure of the cold-rolled steel sheet, that is, the relationship between the ferrite grain size and the punchability. As a result, when the average ferrite grain size in the surface layer portion and the central portion of the sheet thickness within 15% of the sheet thickness from the steel sheet surface is ds and db, respectively, ds / db is controlled to a limited range of 0.95 or less. Has been found to be effective in improving the punching performance. It was also found that the addition of a small amount of B or Nb further improves the punching property.
Based on the above findings, the present inventor has proposed an ultra-low carbon Ti-IF
Based on steel, further added a small amount of B or Nb,
By adjusting the hot rolling conditions, the annealing conditions after cold rolling and the temper rolling conditions to control the metallographic structure of the steel sheet within the above range, the cold rolled steel sheet of the present invention having excellent punchability and the method for producing the same are provided. Heading, the present invention has been completed.

That is, according to the present invention, a cold-rolled steel sheet having excellent punching properties can be obtained by limiting the steel composition, the metal structure and the manufacturing conditions to the following ranges. The reasons for adding the components, the reasons for limiting the components, the reasons for limiting the metal structure, and the reasons for limiting the manufacturing conditions of the present invention will be described below.

(1) Component composition range C: 0.003% or less C is preferably as small as possible to secure the formability of the steel sheet. In practical terms, the upper limit is 0.003% as an amount that does not impair the effects of the present invention. Desirably, the upper limit is 0.0025% for improving the moldability.

Si: 0.3% or less Since Si is a solid solution strengthening element, if added excessively, the strength of the steel sheet increases and the workability deteriorates. For this reason, the upper limit is 0.3%, but preferably 0.15% or less.

Mn: 0.05 to 0.5% Mn is a solid solution strengthening element like Si, and if it is added in excess of 0.5%, the material is deteriorated due to an increase in the strength of the steel sheet. Therefore, the upper limit is 0.5%. Preferably 0.2
% Or less. The lower limit is 0.05% because a steel composition lower than 0.05% leads to an increase in steelmaking cost.

P: not more than 0.03% P, like Si and Mn, is a solid solution strengthening element and increases the strength of the steel sheet by a small amount. Although the punching property is improved by increasing the amount of P, the formability is deteriorated by the increase in strength. Therefore, the upper limit is 0.03%, and preferably 0.015%.
% Or less.

S: 0.003 to 0.015% S is desirably reduced, but if it is less than 0.003%, the releasability of the hot-rolled scale decreases. On the other hand, 0.01
If the content exceeds 5%, the ductility is reduced.
03-0.015%.

Sol. Al: 0.03 to 0.06% Al is necessary for deoxidation, and the lower limit is 0.03%. Also, adding a large amount increases the cost,
0.06% or less. Preferably, it is 0.05% or less.

N: 0.003% or less N is preferably as low as possible in order to ensure high formability of the IF steel, and the upper limit is 0.003%. Desirably, it is 0.0025% or less. Ti: 0.02 to 0.08% Ti is a strong carbonitride forming element, and when added to extremely low carbon steel, the formability of the steel sheet is improved. Ti-I
In order to obtain the high deep drawability unique to F steel, at least 0.02% is necessary, and this value is the lower limit. On the other hand, adding more than 0.08% not only increases the slab cost,
Since the surface properties of the cold-rolled steel sheet are likely to deteriorate, the upper limit is set to 0.
08%.

B: 0.0002 to 0.0015% B is added in combination with Ti to improve punchability. It is also possible to improve the vertical cracking resistance as a secondary effect, and it is possible to improve the inevitable secondary working brittleness of Ti-IF steel. When the content is 0.0002% or more, the further effect of B on refining the surface layer of the steel sheet is recognized, so the lower limit is 0.0002%. Further, if the content exceeds 0.0015%, the deep drawability of the steel sheet itself is likely to deteriorate, so the upper limit is 0.0015%. Nb: 0.005 to 0.03% Nb is also a carbonitride forming element like Ti, and when added to ultra-low carbon steel, the formability of the steel sheet is improved and the surface layer of the steel sheet is refined by Nb. Since the effect is expected, the punchability is further improved as compared with Ti-IF steel.
In the case where Ti is added in combination with Ti, excessive addition of Nb not only increases slab cost but also leads to deterioration of ductility, so the upper limit is 0.03%. On the other hand, if the addition is less than 0.005%, the effect of improving the punching property by Nb cannot be obtained, so the lower limit is 0.005%.

(2) Metal Structure The metal structure of the present invention is such that when the average ferrite grain size in the surface layer portion and the center portion of the sheet thickness within 15% of the sheet thickness from the surface of the steel sheet is ds and db, respectively, ds / db is 0. .95 or less. If ds / db exceeds 0.95, good punching characteristics cannot be obtained.

In the present invention, as the microstructure of the annealed steel sheet for improving the punchability of the cold-rolled steel sheet, fine precipitation is performed as a precipitation form of TiC on the surface layer of the steel sheet, and the ferrite structure near the surface layer is removed from the inner layer. By forming a fine structure, the surface layer is hardened from the inner layer in terms of the material, thereby improving the punching property.

Therefore, the average ferrite grain size in the surface layer portion and the center portion of the sheet thickness within 15% of the sheet thickness is ds and d, respectively.
When b is set, it is important that ds / db is 0.95 or less. The lower limit value of ds / db is not particularly limited, but the minimum value of ds / db actually obtained by the present invention is around 0.4.

The importance of ds / db has been clarified by experiments of the present invention. That is, using the steel which satisfies the steel composition described in claim 1, cold rolling after cold rolling after hot rolling by the method according to claim 4 and a conventional method, and performing a heat treatment of a continuous annealing image. Prepare steel sheet, ds / db
Was obtained in various ways. Using this cold-rolled steel sheet, the relationship between the punching property of the cold-rolled steel sheet and ds / db is as follows.
A survey was conducted. In addition, as an evaluation method of the punching property,
A 0.8 mm-thick cold-rolled steel plate is punched with a punch of 10 mmφ and a die of 10.3 mmφ, and the burr height around the punched hole is measured with a point micrometer. The punchability was evaluated according to the magnitude of the size. FIG. 1 is a diagram showing the relationship between ds / db and burr height. As shown in FIG. 1, it can be understood that the punching performance can be improved by controlling ds / db to 0.95 or less.

By adjusting the composition range and the metal structure as described above, it is possible to obtain a cold rolled steel sheet having excellent punching properties. Steel having such characteristics can be manufactured by the following manufacturing method.

(3) Steel plate manufacturing process After the steel adjusted to the above-mentioned component composition range is melted in a converter, it is made into a slab by continuous casting.
Heat to

At a heating temperature higher than 1200 ° C., almost all of the TiC, TiS, and Ti ₄ C ₂ S ₂ precipitates in the slab form a solid solution. No good deep drawability can be obtained. 1000 ° C
At heating temperatures less than, TiC, TiS, Ti ₄ C ₂ S ₂
Because the precipitate does not re-dissolve in the heating stage and remains as a coarse precipitate during solidification of the slab, good deep drawability can be obtained, but punching, which is an inevitable drawback of IF steel, is obtained. Sex is not improved at all. Accordingly, the heating temperature is 1000 to 1200 ° C., and in order to improve the punching property of a cold-rolled steel sheet while maintaining a coarse precipitation form in some of the precipitates, some of the precipitates are re-solidified by the heating. It is important to dissolve.

In the case of direct rolling, in which hot rolling is performed as it is after solidification, TiC, TiS, Ti ₄ C ₂ S ₂ precipitated in steel
Precipitates other than TiN tend to be finer as a whole,
As a result, good deep drawability unique to IF steel cannot be obtained, so that this slab heat treatment is an essential condition. In addition, the desirable range of the heating temperature after continuous casting for improving the punching property is 1000 to 1150 ° C.

Next, the heated steel is subjected to final rough rolling in a temperature range of not more than ₃ points of Ar.
C is the rough rolling final pass by rolling at Ar ₃ point or less of the temperature range, the rough rolling a slab surface layer portion became Ar ₃ point or less of the temperature and strain-induced fine precipitation by rolling strain. With the finely precipitated TiC, a fine ferrite structure can be obtained in the surface layer of the slab. Therefore, after the rough rolling, simply α
Cooling to a region is meaningless for fine precipitation, since no strain-induced precipitation occurs.

Thereafter, the roughly-rolled steel is heated to a temperature range of not less than the Ar ₃ point and less than (Ar ₃ +100) ° C. at a heating rate of 5 ° C. /
s or more, desirably a temperature difference of 30 ° C. or more, and Ar ₃
Finish rolling at above the point and winding.

When the heating temperature is set to (Ar ₃ +100) ° C. or higher and when the heating rate is set to less than 5 ° C./s, the finely precipitated TiC is dissolved again in any case.
For this reason, the precipitation form of TiC in the hot-rolled sheet cannot be made into a fine precipitation state only in the surface layer portion, the surface layer portion does not have a fine ferrite structure, and the effect of the present invention cannot be sufficiently obtained.

Note that the upper limit of the heating rate is not particularly limited as long as there is no problem in controllability or the like in heating.
In addition, the amount of temperature rise in heating after rough rolling is desirably 30 ° C. or more. This is because if the temperature is lower than 30 ° C., the effect of retransforming only the surface layer from α to γ is not sufficient, and as a result, the surface layer may not have a fine ferrite structure. This is because the effect may be reduced.

Finally, with respect to terminating finish rolling at _three or more Ar points, if finishing at less than _three Ar points,
Since good deep drawability unique to F-steel cannot be obtained, finish rolling needs to be completed at _three or more Ar points.

Under the above-described limited conditions, only the surface layer of the rough rolled material is transformed from γ to α, and reheated to Ar ₃ or more at a heating rate of 5 ° C./s or more, so that only the surface layer has α From γ to γ. By transforming the γ → α → γ transformation and the fine TiC received by the surface layer of the rough rolled material in a concentrated manner on the surface layer, the surface layer of the cold rolled steel sheet can have a fine ferrite structure.

The winding temperature is not particularly limited, but is preferably set to 600 to 700 ° C. in order to secure the high deep drawability unique to IF steel. Next, the rolled hot-rolled steel sheet is cold-rolled, annealed at a recrystallization temperature of 850 ° C., and temper-rolled at a pressure control rate of 1.5% or less. Annealing is performed continuously at a recrystallization temperature of 850 ° C. or lower. 85
When the temperature exceeds 0 ° C., C is re-dissolved in the γ phase to cause a decrease in elongation. On the other hand, when the temperature is lower than the recrystallization temperature, the crystal grains are fine, a good texture is not developed, the r value is low, and the formability is poor. The temper rolling is performed at a pressure control rate of 1.5% or less for the purpose of controlling the surface roughness of the cold-rolled steel sheet. Here, when the pressure regulation ratio exceeds 1.5%, the material such as elongation deteriorates, and therefore the pressure regulation ratio is 1.5% or less.

Further, 0.0002 to 0.0015% of B
And the addition of 0.005 to 0.03% Nb to form a solid solution of B and Nb in steel,
(C, N) further refines the surface layer of the steel sheet in the recrystallization stage after cold rolling, so that the combined addition of B and Nb is effective for further improving the punching property.

The steel sheet of the present invention is subjected to continuous galvanizing of a steel sheet which has been subjected to various electroplating treatments and an organic coating treatment and a cold-rolled steel sheet after continuous annealing to improve corrosion resistance. Including surface-treated steel sheets,
The effect of the present invention can be sufficiently obtained with these steel sheets.
Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0037]

【Example】

(Example 1) Table 1 shows the compositions of the steels of the present invention (Nos. 1 to 15) and comparative steels (Nos. 16 to 21) produced in a laboratory melting furnace. From these steels, slabs for laboratory hot rolling were prepared, and hot rolled, pickled, cold rolled, and continuously annealed by ordinary methods to obtain cold rolled steel sheets. Samples for structure observation and samples for punching evaluation were collected from these cold-rolled steel sheets, and the structure observation, punchability, and mean-r value were investigated. The method for evaluating the punching property was the same as the method described above.

The mean-r value was determined by measuring the JIS No. 5 tensile test piece in the rolling direction of the steel sheet, in a direction at 45 degrees with respect to the rolling direction, at 90 degrees.
After sampling in the degree direction and measuring the respective r values, r ₀ , r ₄₅ , and r ₉₀ with a pre-strain of 15%, the mean-r =
It was calculated by (r ₀ + 2r ₄₅ + r ₉₀ ) / 4.

Table 1 shows the results of microstructure observation (d
s / db), burr height and mean-r value. Table 1
From the results, it is clear that the steel No. of the present invention satisfying the steel composition of the present invention and satisfying the ferrite grain size. In Nos. 1 to 15, a good punching property with a burr height of 0.03 mm or less is obtained, and also a good deep drawing property is provided.

On the other hand, the comparative steel No. 16 satisfies the composition of the present invention but has a ds / db value of more than 0.95.
The punchability is not good. In addition, No. In the comparative steels Nos. 17 to 21, the elements C, Ti, Nb, and B did not satisfy the scope of the present invention. In this case, since the ferrite structure becomes uniform over the entire thickness, the ds / db value exceeds 0.95, and the punching property is deteriorated. Regarding the deep drawability, the steel No. It is understood that a high mean-r value peculiar to the IF steel cannot be obtained except for No. 18.

[0041]

[Table 1]

(Example 2) Steel No. 1 in the steel of the present invention shown in Table 1 was used. Four types 1, 4, 8, and 12 were selected, and hot rolling was carried out under various conditions using a laboratory cast slab having these components. Then, after performing pickling and cold rolling according to a conventional method, a heat treatment simulating continuous annealing was performed at 830 ° C., and finally, 1% temper rolling was performed. Thereafter, a sample for structure observation and a sample for punchability evaluation were collected from the obtained cold-rolled steel sheet and subjected to various accuracy tests.

As shown in Table 2, (1) the slab heating temperature is 1000 to 1150 ° C., and (2) the final pass of the rough rolling is Ar.
₃ points or less, (3) heating rate of rough rolled material is 5 ° C / s or more,
(4) The ultimate temperature of heating of the rough rolled material is Ar ₃ or more (Ar ₃
Comparative Example No. 5 which is less than (+100) ° C. and (5) the finish rolling temperature does not satisfy any one of the production conditions of the present invention in which Ar is ₃ points or more. 2-5,7-10,12-15,1
In the range of 7 to 20, the value of ds / db does not satisfy 0.95 or less. For this reason, the burr height is a high value exceeding 0.04 mm, and the punching property is not good. On the other hand, (1)
Inventive Example No. 5 satisfying all the conditions of (1) to (5). 1,
In Nos. 6, 11, and 16, it is clear that a cold rolled steel sheet having an extremely excellent punching property with a ds / db value of 0.95 or less and a burr height of 0.03 mm or less can be obtained.

From the above, it is understood that satisfying all the production conditions of the present invention (1) to (5) is an important production factor for imparting excellent punchability to an IF steel cold-rolled steel sheet. Is shown.

[0045]

[Table 2]

[0046]

According to the present invention, it is possible to provide various IF steel cold-rolled steel sheets having excellent punching characteristics without deteriorating the material by specifying the steel composition, metal structure, and manufacturing conditions. In addition, an industrially superior effect is brought.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between ds / db and burr height according to an embodiment of the present invention.

Claims (6)

[Claims] 1. The method according to claim 1, wherein C: 0.003% or less in weight%,
i: 0.3% or less, Mn: 0.05 to 0.5%,
P: 0.03% or less, S: 0.003 to 0.015%
And Sol. Al: 0.03-0.06%, Ti:
0.02 to 0.08% and N: 0.003% or less, in the surface layer portion having a thickness of 15% or less from the surface of the steel sheet and in the central portion of the thickness of the sheet excluding the thickness of 15% or less from the steel surface When the average ferrite grain size is ds and db, respectively, ds /
A cold rolled steel sheet having excellent punchability, wherein db is 0.95 or less. 2. B: 0.0002-% by weight
The cold-rolled steel sheet having excellent punchability according to claim 1, wherein the steel sheet contains 0.0015%. 3. Nb: 0.005 to 5% by weight.
The cold rolled steel sheet having excellent punchability according to claim 1 or 2, which contains 0.03%. 4. The method for producing a cold-rolled steel sheet according to claim 1, wherein the continuously cast steel is heated to 1000 to 1200 ° C., and the heated steel is subjected to a final roughing process in a temperature range of ₃ points or less of Ar. Rolling and re-heating the roughly-rolled steel at a heating rate of 5 ° C./s or more to a temperature range of not less than Ar ₃ points and less than (Ar ₃ +100) ° C.
Finish rolling at _three or more points, winding step, cold rolling of hot rolled steel sheet, annealing at recrystallization temperature ~ 850 ° C, and temper rolling at 1.5% or less pressure control rate A method for producing a cold-rolled steel sheet having excellent punching characteristics, comprising: 5. The steel sheet further comprises B: 0.0% by weight.
The method for producing a cold-rolled steel sheet having excellent punchability according to claim 4, which contains 002 to 0.0015%. 6. The steel sheet may further contain Nb: 0.
The method for producing a cold-rolled steel sheet having excellent punchability according to claim 4, wherein the method comprises 005 to 0.03%.