JPH0762209B2 - Cold rolled steel sheet having excellent composite press formability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a cold-rolled steel sheet having excellent composite press formability and having TS ₄₅ of steel sheet having a characteristic value of 28.5 to 31.0 kgf / mm ² and r ₄₅ of 1.90 or more. , And a method for producing the steel sheet by continuous annealing.

(Prior art) Regarding the technology to improve the press formability of cold rolled steel sheet,
A method of box annealing with step heating of a super workable cold-rolled steel sheet having a total elongation of 54% or more and a value of 2.0 or more is disclosed in JP-A-63-21.
Japanese Patent Laid-Open No. 61-113724 discloses a method for manufacturing a steel plate having an extremely high value or r ₄₅ and El.

Recently, many pressed parts for automobiles have complicated shapes such as a wheel house inner, and the formability thereof has become extremely difficult. The present inventors also
With respect to these pressed parts, the method for producing steel plates with extremely high values and r ₄₅ and El previously invented by the present inventors is disclosed in Japanese Patent Laid-Open No. Sho 61-61.
Although steel sheets manufactured by applying the method of Japanese Patent Laid-Open No. 113724 have been supplied to automobile manufacturers, the problem of frequent press defects has arisen.

(Problems to be solved by the present invention) The problem to be solved by the present invention is that TS ₄₅ of a steel plate is 28.5
It is to provide a cold-rolled steel sheet having excellent composite press formability having a characteristic value of 31.0 kgf / mm ² , r ₄₅ of 1.90 or more, and a method for producing the steel sheet by continuous annealing.

(Means for Solving the Problems) The inventors of the present invention have realized the realization of these parts (hereinafter referred to as “composite molded parts”) having a deep drawing and an overhang forming and a drawing bead having a complicated shape such as a wheel house inner. In press, development of a cold-rolled steel sheet that has excellent press formability (hereinafter referred to as composite press formability) and a method for producing the steel sheet by continuous annealing are earnestly studied,
The following results were found.

The gist of the present invention is as follows.

(1) C ≦ 0.0025%, Si ≦ 0.05%, Mn ≦ 0.30%, P: 0.007
~ 0.030%, S ≦ 0.020%, solAl ≦ 0.080%, N ≦ 0.0030%, T
i: 0.025 to 0.120%, Nb: 0.003 to 0.020%, B ≦ 0.0004%, balance unavoidable impurities and iron.
Tensile strength in the ° direction (hereinafter referred to as TS ₄₅ ) is 28.5 to 31.0k
gf / mm ² , r value in rolling direction and 45 ° direction (hereinafter referred to as r ₄₅ )
Cold rolled steel sheet with excellent composite press formability of 1.90 or more.

(2) C ≦ 0.0025%, Si ≦ 0.05%, Mn ≦ 0.30%, P: 0.007
~ 0.030%, S ≦ 0.020%, solAl ≦ 0.080%, N ≦ 0.0030%, T
i: 0.025 to 0.120%, Nb: 0.003 to 0.020%, B ≦ 0.0004%, the remaining inevitable impurities and a steel slab composed of iron are heated to 880
Finish hot rolling at ~ 940 ° C, start quenching within 1.5sec, cool at least 850 ° C at a cooling rate of 50 ~ 200 ° C / sec, wind at 680 ~ 770 ° C, hot rolled steel strip And 70
Characterized by performing cold rolling at a cold rolling rate of not less than%, and then performing recrystallization annealing at 750 to 900 ° C. in continuous annealing,
The tensile strength TS _{45 of the} steel plate rolling direction and 45 ° direction is 28.5 to 3
A method for producing a cold-rolled steel sheet having excellent composite press formability, which has a characteristic value of 1.0 kgf / mm ² , an r value r ₄₅ in the rolling direction and a 45 ° direction of 1.90 or more.

The present invention will be described in detail below.

First, the first invention will be described. The inventors of the present invention have demonstrated excellent press formability in actual pressing of deep draw forming and overhang forming having a complicated shape such as a wheel house inner and those parts having draw beads (hereinafter referred to as composite formed parts), for example. Hereinafter, in order to develop a cold-rolled steel sheet capable of obtaining composite press formability), first, El = 51.0, = 2.52 introduced in the Iron and Steel Institute of Japan Vol.3 No.6 1990 P.1771. , including cold-rolled steel sheet having excellent material as r ₄₅ = 2.40, apply a cold rolled steel sheet with excellent material in El ,, r ₄₅ the composite molded part investigated the actual press formability. As a result, the above El = 51.0, = 2.52,
It is considered possible from the conventional knowledge that many actual press-formability products were produced in spite of the good material of El ,, r ₄₅ , including the cold-rolled steel sheet having an extremely excellent material of r ₄₅ = 2.40. No results.

Therefore, as a result of detailed examination of the relationship between these composite press formability and material property values, the present inventors have found that the T.
S ₄₅ has a characteristic value of 28.5 to 31.0 kgf / mm ² , r ₄₅ is 1.90 or more, and further, the B content is 0.0004% or less, preferably 0.
It has been found that a steel sheet having a content of 0002% or less can obtain excellent composite press formability. Since the cold-rolled steel sheet introduced in the above-mentioned Proceedings of the Iron and Steel Institute of Japan Vol.3 No.6 1990 P.1771 was prototyped for the oil pan of automobiles, in order to prevent secondary working brittleness after each cylinder forming, B Since it was a cold-rolled steel sheet containing 0.0008% of Al, it is considered that the actual press composite press formability was poor as described above.

FIG. 1 shows the results of a detailed study of the relationship between the above-mentioned composite press formability and material property values.
A steel sheet having TS ₄₅ of 28.5 to 31.0 kgf / mm ² and r ₄₅ of 1.90 or more and a B content of 0.0004% or less, preferably 0.0002% or less has excellent composite press formability. Is obtained, the steel sheet of the present invention, for example, in the actual press of these parts having a deep drawing and a stretch forming and a drawing bead having a complicated shape such as a foil house inner, excellent press formability is obtained, You can see that it has excellent industrial value.

As described above, it is difficult to clearly clarify the reason why the steel sheet of the present invention has excellent composite press formability,
It is considered as follows.

(1) The reason why TS _{45 of} steel sheet is 28.5 to 31.0 kgf / mm ² is indispensable is that, for example, deep drawing forming and overhang forming with complicated shapes such as a wheel house inner and those parts having drawing beads are used. The forming limit in the actual press is the draw bead part and the deep draw forming part (The most severe deep drawing of most deep drawn parts is the rolling direction of the steel plate.
It is assumed that it is almost determined by the friction resistance when the steel plate flows in at the die shoulder R part (which coincides with the 45 degree direction) and the magnitude of the breaking strength (TS ₄₅ ) of the steel plate. Therefore, TS ₄₅ is 28.5kgf
It seems that over / mm ² has become essential for these composite molded parts of automobiles.

If TS ₄₅ exceeds 31.0 kgf / mm ² , the ductility of the steel sheet may deteriorate.

(2) The reason why r ₄₅ needs a characteristic value of 1.90 or more is that the deep drawability in the direction of 45 ° is required as described above, but it is disclosed in, for example, Japanese Patent Application Laid-Open No. 61-113724. At a level of r ₄₅ ≦ 1.85, good composite press formability cannot be obtained.

(3) The reason why the content is required to be 0.0004% or less, preferably 0.0002% or less is not always clear, but according to the results of various studies by the present inventors, Many cold-rolled steel sheets containing 0.0005% or more of B had the same local elongation value in the tensile test, but only low values could be obtained even when compared. Therefore, such a steel sheet having a local elongation value that is low relative to TS is subjected to severe bending back processing when the steel sheet flows into the die bead portion or the die shoulder R portion of the deep drawing portion, and the local elongation value is low. Therefore, it is presumed that the workability deteriorated so much that press rupture easily occurred.

Furthermore, the effects described in (1), (2), and (3) above are
Each item is effective, but only after all are satisfied,
The present invention provides a cold-rolled steel sheet having excellent composite press formability, which is the object of the present invention. C ≦ 0.0040%, Si ≦ 0.05%, Mn ≦ 0.
30%, P ≦ 0.030%, S ≦ 0.025%, solAl ≦ 0.100%, N ≦ 0.005
0%, Ti: 0.025 to 0.120%, Nb: 0.003 to 0.020%, B ≤ 0.0004
%, The balance unavoidable impurities and iron as the cold-rolled steel sheet is a base component for industrially and economically obtaining a cold-rolled steel sheet having excellent composite press formability that satisfies the above characteristics. Is.

Hereinafter, the second invention will be described. The second invention is a method for producing the cold-rolled steel sheet according to the first invention by a continuous annealing method. The point of the method of the second invention is to comprehensively adjust the production conditions so that the characteristic values of the obtained cold-rolled steel sheet match the characteristic values of the first invention, and produce a cold-rolled steel sheet satisfying the first invention conditions. By providing a method for manufacturing cold-rolled steel sheets with excellent composite press formability that has never been seen before, the steel composition, hot rolling conditions, cold rolling, and continuous annealing conditions are regulated to specific conditions, and the steel sheet TS ₄₅ is 28.5 ~ 31.0k
It is an object of the present invention to provide a method for producing a cold-rolled steel sheet having excellent composite press formability and having a characteristic value of gf / mm ² , r ₄₅ of 1.90 or more.

The manufacturing conditions are described in detail below.

C is an extremely important element in obtaining a cold rolled steel sheet having a characteristic value of r ₄₅ of 1.90 or more by continuous annealing, and when the C content exceeds 0.0025%, these characteristic values cannot be obtained. The upper limit was made 0.0025% or less.

When the content of Si, Mn, S, and N increases, r ₄₅ decreases, and it becomes difficult to obtain a characteristic value of 1.90 or more. Therefore, the content of each of Si, Mn, S, and Mn is 0.30%, S ≦ 0.020%, N ≦ 0.00
30%.

If P is less than 0.007%, it becomes difficult to obtain TS ₄₅ of 28.5 kgf / mm ² . Further, if it exceeds 0.030%, the deterioration of the secondary workability becomes too large, so the content was made 0.030% or less.

Although solAl is generally used as a deoxidizing agent, in the case of the present steel, there is no problem even if Ti deoxidizing is performed, so it is not necessary to specify the lower limit value. When the content of solAl increases, r
_{Since 45} decreases, it becomes difficult to obtain a characteristic value of 1.90 or more,
Each content was set to 0.080% or less.

Ti is an extremely important element in obtaining an r ₄₅ value of 1.90 or more, and when the Ti content is less than 0.025%, this characteristic value cannot be obtained. If the Ti content exceeds 0.120%, TS ₄₅ will be 31.0.
Since it will exceed kgf / mm ² , the Ti content will be 0.025% ~
Restricted to 0.120%.

Nb is an extremely important element over Ti in obtaining an r ₄₅ value of 1.90 or more, and when the Nb content is less than 0.003%, this characteristic value cannot be obtained. On the other hand, if the Nb content exceeds 0.020%, the recrystallization temperature becomes high and the crystal grains become finer, resulting in TS ₄₅ of 31.0 kgf / mm ²
The Nb content is 0.003% to 0.020%.
Regulated to.

B is an element that significantly improves the secondary workability, but at the same time, as described above, cold-rolled steel sheets containing 0.0005% or more of B have the same local elongation value in the tensile test even if they are compared in TS. The B content must be 0.0004% or less, preferably 0.0002% or less, since only a low value can be obtained and good composite press moldability cannot be obtained.

The hot rolling condition is a step of rendering C as TiC harmless and making the crystal grains of the hot rolled sheet finer by the combined addition effect of 0.025% or more Ti and 0.003% or more Nb.

The heating condition before hot rolling may be a heating condition that is usually performed without any particular regulation, but in order to obtain a softer steel plate, 11
A heating temperature of 00 ° C. or lower (referred to as SRT) is preferable.

When the finishing temperature of hot rolling is less than 880 ° C, coarse crystal grains are formed, and the r ₄₅ value is significantly reduced. Further, even if the temperature exceeds 940 ° C, the crystal grains become too large and an r ₄₅ value of 1.90 or more cannot be obtained, so it is necessary to set the temperature to 880 to 940 ° C.

The coiling temperature (hereinafter referred to as CT) is an important step for precipitating and coarsening more TiC and obtaining an excellent r ₄₅ value. At a temperature below 680 ° C, precipitation of TiC and coarsening become insufficient, and an excellent r ₄₅ value cannot be obtained. Also, when the temperature exceeds 770 ° C, coarsening of crystal grains occurs, and r _{45 of} 1.90 or more occurs.
Since the value cannot be obtained, it is necessary to set the temperature to 680-770 ° C.

Further, at least 1.5 seconds after finish rolling and at a cooling rate of 50 ° C./s or more for refining the crystal grains of the hot rolled sheet.
It is necessary to quench down to 850 ℃. From this condition, if the rapid cooling start time is slow, or if the rapid cooling temperature range or cooling rate is insufficient, the crystal grains become large and r _{45 of} 1.90 or more.
No value can be obtained.

If the cold rolling ratio is less than 70%, the r ₄₅ value decreases, and the cold rolled steel sheet aimed at by the present application cannot be obtained. Therefore, the cold rolling ratio was set to 70% or more.

As for the continuous annealing conditions, if recrystallization annealing is performed at an annealing temperature of 750 ° C. to 900 ° C., the cold-rolled steel sheet intended by the present application can be manufactured, and it is not necessary to regulate other conditions. If the annealing temperature is less than 750 ° C, an r ₄₅ value of 1.90 or more cannot be obtained, and if the annealing temperature exceeds 900 ° C, the crystal grains become abnormally large and TS _{45 of} 28.5 kgf / mm ² becomes difficult to obtain. Conditions were regulated to 750-900 ° C.

Temper rolling does not have to be particularly restricted because it may be performed as needed.

Incidentally, the present invention is not limited to cold-rolled steel sheet, electrogalvanized,
It can also be applied as a method for producing an original plate of a surface-treated steel plate such as tin plating, aluminum plating, and turn plating. Furthermore, even if the present invention is applied to a continuous annealing hot dip galvanizing method that is usually performed, if the recrystallization annealing conditions before hot dip galvanizing are within the condition range of the present invention, TS _{45 of the} steel sheet is 28.5 to 31.0 kgf / mm. ² , r ₄₅
It is possible to produce a hot-dip galvanized steel sheet having a characteristic value of 1.90 or more and excellent in composite press formability. Note that the material characteristic values of these surface-treated steel sheets are affected by the material characteristic values such as the hardness of the metal or alloy plated on the surface of the steel sheet and the plating thickness, and usually Y, P, T, S are It becomes high and the El, r value decreases.
However, the press formability of the surface-treated steel sheet is determined not by the characteristic value of the plated steel sheet including the skin but by the material of the base material of the steel sheet. Therefore, in the present invention, it is necessary that the characteristic value after plating peeling satisfies the conditions of the cold rolled steel sheet of the present invention, and in such a case, excellent composite press formability can be obtained.

(Example) Below, the effect of this invention is demonstrated with an Example.

The components shown in Table 1 and the hot rolling conditions shown in Table 2 (however, the finishing temperature was 910 to 930 ° C) were used to produce 4.0 mm hot rolled steel strips, and cold rolled 0.8 mm cold rolled steel sheets were continuously produced. 820 ℃ × 60sec by annealing
After recrystallization annealing and cooling to room temperature, 0.8% temper rolling was performed to obtain a cold rolled steel sheet.

Characteristic values of mechanical properties of the obtained cold rolled steel sheet (TS ₄₅ , El ₄₅ , r
₄₅ values) were measured and the results are shown in Table 2.

Further, in order to investigate the actual press formability of the composite molded part, an actual press test of the wheel house inner was conducted, and the press defect rate was investigated, and the results are shown in Table 2.

Sample No. 10 is an example of the present invention in which Zn-Ni electroplating (30 g / mm ² ) was performed, and the mechanical properties are characteristic values after the plating was peeled off.

Steels A, B, and E are steels having a composition outside the scope of the method of the present invention.
C, D, and F are production conditions within the range of components of the method of the present invention.

Samples 3, 4, 6 and 10 are examples of the method of the present invention, and in all cases, TS ₄₅ required as a cold rolled steel sheet having excellent composite press formability was 2
Characteristic values of 8.5 to 31.0 kgf / mm ² , r ₄₅ of 1.90 or more were obtained, and excellent results of press defect rate of 0.5% or less were obtained.

Samples 1,2,5,7,8,9 were the conventional examples or the comparative examples, and all had a press defect rate of 2.2% or more.

Sample 1 is a conventional example of a component system of a normal Ti-added ultra-low carbon steel containing no Nb. The obtained property values were insufficient for TS ₄₅ and r ₄₅ , and the press defect rate was 100%, which means that it could not be narrowed down at all.

Sample 2 is a comparative example in which P is 0.003%, which is outside the range of the present invention, and the obtained characteristic values are mechanical properties El ₄₅ and r _{45 which} are the most excellent characteristic values among the examples. It was However, the press defect rate of the actual press of the wheel house inner was 4.2%, which was a bad press result. This is because the actual pressability of a composite molded part such as a wheelhouse inner is indispensable for TS _{45 to} have a characteristic value of 28.5 to 31.0 kgf / mm ² and r ₄₅ of 1.90 or more, like the steel sheet of the present invention. I understand that there is.

Samples 3, 4, and 6 are examples in which the components and hot rolling conditions were changed within the scope of the present invention as shown in Tables 1 and 2. If the manufacturing conditions are within the scope of the present invention, TS ₄₅ required as a cold rolled steel sheet having excellent composite press formability is 28.5 to 31.0 kgf / mm ² , r
_It can be seen that ₄₅ has a characteristic value of 1.90 or more, and the press defect rate of the steel sheet is 0.5% or less, which is an excellent result.

Sample 5 is a comparative example in which B is as high as 0.0008%, and although TS ₄₅ and r ₄₅ of the obtained steel sheet are within the range of the mechanical properties of the steel sheet of the present invention, the defective press rate is extremely poor at 3.8%. It is the result. It is considered that this is because the local elongation value in the tensile test of the steel plate decreased because B was as high as 0.0008%. Therefore, it is understood that it is important to set the B content to 0.0004% or less.

Samples 7, 8 and 9 are comparative examples in which the hot rolling conditions were out of the range of the present invention as shown in Table 2. Sample 7 had tQ and sample 8 had α.
CT of the sample 9 is off, and the r ₄₅ value of each is insufficient, which is less than 1.90. The defective press rate is also worse than 2.2%. FIG. 1 shows the comprehensive results of these experiments. According to FIG. 1, the range (shaded portion) of the present invention is TS ₄₅ of 28.5 to 31.
Characteristic values of 0kgf / mm ² and r ₄₅ of 1.90 or more were obtained, and the press defect rate of the steel sheet was also within the excellent range of 0.5% or less, but those with a high B mark + are TS ₄₅ and r ₄₅ Although the invention is satisfied, the defective press rate is high and does not belong to the present invention.

Sample No. 10 is an example of the present invention in which Zn-Ni electroplating (30 g / mm ² ) was performed. If the characteristic values after stripping the plating satisfy the conditions of the steel sheet of the present invention, an excellent composite press It can be seen that moldability is obtained.

(Effects of the Invention) Although the present invention has been described in detail above, the present invention (1)
The cold-rolled steel sheet of No. 1 has excellent composite press formability, and according to the present invention (2), TS _{45 of} the steel sheet is 28.5 to 31.0 kgf / mm ² , r _45.
It becomes possible to manufacture a cold-rolled steel sheet having excellent composite press formability having a characteristic value of 1.90 or more, and its industrial value is great.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between composite press formability and material property values.

Front page continued (72) Inventor Tomohiro Nono 1 Fuji-machi, Hirohata-ku, Himeji-shi, Hyogo Shin-Nippon Steel Corporation Hirohata Works (72) Inventor Hiroshi Kubo 1-fuji, Hirohata-ku, Himeji-shi, Hyogo Nippon Steel Co., Ltd., Hirohata Works (56) References JP-A-63-76848 (JP, A)

Claims (2)

[Claims] 1. C ≦ 0.0025%, Si ≦ 0.05%, Mn ≦ 0.30%, P:
0.007 to 0.030%, S ≦ 0.020%, solAl ≦ 0.080%, N ≦ 0.0030
%, Ti: 0.025 to 0.120%, Nb: 0.003 to 0.020%, B ≦ 0.0004
%, The balance is unavoidable impurities and iron, and the tensile strength in the rolling direction and 45 ° direction of the steel sheet (hereinafter referred to as TS ₄₅ ) is 28.5.
A cold-rolled steel sheet having an excellent composite press formability of up to 31.0 kgf / mm ² and an r value in the rolling direction and the 45 ° direction (hereinafter referred to as r ₄₅ ) of 1.90 or more. 2. C ≦ 0.0025%, Si ≦ 0.05%, Mn ≦ 0.30%, P:
0.007 to 0.030%, S ≦ 0.020%, solAl ≦ 0.080%, N ≦ 0.0030
%, Ti: 0.025 to 0.120%, Nb: 0.003 to 0.020%, B ≦ 0.0004
%, The balance unavoidable impurities and steel slabs are heated, hot finish rolling is completed at 880 to 940 ℃, quenching is started within 1.5 seconds, and at least 850 ℃ up to 50 to 200 ℃ / sec. It is cooled at a cooling rate, wound at 680-770 ° C, made into a hot-rolled steel strip, cold-rolled at a cold rolling rate of 70% or more, and then recrystallized by continuous annealing at 750-900 ° C. Characteristically, the tensile strength TS _{45 in} the rolling direction of the steel plate and in the 45 ° direction is 2
8.5〜31.0kgf / mm ² , r value r _{45 in} rolling direction and 45 ° direction is 1.90
A method for producing a cold-rolled steel sheet having the above characteristic values and excellent in composite press formability.