JPH11100637A - Cold rolled steel sheet for deep drawing, having non-aging characteristic and excellent in baking finish hardenability, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably provide a cold rolled steel sheet for deep drawing, having non-aging characteristic, by means of low temp. annealing and further to stably provide, by means of low temp. annealing, a cold rolled steel sheet for deep drawing, having non-aging characteristic and excellent in baking finish hardenability, by performing overaging treatment under proper conditions. SOLUTION: In a cold rolled steel sheet for deep drawing, having non-aging characteristic, C content is <=0.0020 wt.% and the atomic ratio between B content and N content, B/N, is regulated to 0.6-1.4 and also the ratio between the amount of AlN precipitate and that of BN precipitate, AlN/BN, is regulated to 0.05-1.0. Further, the cold rolled steel sheet for deep drawing, having non- aging characteristic and excellent in baking finish hardenability, can be produced by subjecting the cold rolled steel sheet, which has 0.0012-0.0020 wt.% C content and in which the other chemical components are regulated in the same way as above, to recrystallization annealing and then to overaging treatment at 250-500 deg.C for >=30 sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-ageing deep drawn cold-rolled steel sheet having a low recrystallization temperature.

[0002]

2. Description of the Related Art Conventionally, ordinary cold-rolled steel sheets have been manufactured by batch box annealing. However, as continuous annealing lines have been expanded in recent years, over-aging treatment is not required and extremely low formability is achieved. Carbon steel sheet (usually C is 0.005% by weight or less)
This ultra-low carbon steel sheet, which is becoming the mainstream of cold-rolled thin steel sheets, was first invented as a Ti-added ultra-low carbon steel sheet that requires Ti addition, and even now it is an ultra-low carbon steel sheet due to its excellent formability and non-aging properties. Is used as a typical steel type. In addition, Nb-added ultra-low carbon steel sheet with improved hot-dip coating properties, which is the only weak point of Ti-added ultra-low carbon steel sheet, and N
Ultra-low carbon steel sheets and the like to which b and Ti are added in combination have been developed and put to practical use.

[0003] These ultra-low carbon steel sheets require less construction cost for continuous annealing equipment than ordinary low carbon steel sheets (C content is about 0.05% by weight), as described above, because overaging treatment is not required as described above. Although there are advantages such as lower cost, the high recrystallization temperature increases the annealing cost, and at the same time, deteriorates the sheet passing property (for example, heat buckle) to cause a decrease in yield. The recrystallization temperature of a low carbon steel sheet is usually about 650 ° C., whereas that of a Ti-added ultra low carbon steel sheet is about 750 ° C.
In the case of the b-added extremely low carbon steel sheet, the temperature is even higher than 800 ° C. Therefore, in order to lower the recrystallization temperature of the ultra-low carbon steel sheet, for example, Japanese Patent Application Laid-Open No.
And the recrystallization temperature is lowered.
However, the cost of steel making for extremely low P increases greatly, and is not always practical.

In a cold rolled steel sheet used as a steel sheet for automobiles, as a means for increasing the dent resistance, a coating in which the undulation point rises due to the strain age hardening of interstitial elements during the coating baking treatment of the automobile after pressing. Bake hardening properties (hereinafter abbreviated as BH properties) are used. As means for obtaining the BH property, a technique of adjusting the amount of solid solution C by adding Ti or Nb alone or in combination with ultra-low carbon steel to secure the BH property is disclosed in JP-A-53-114717 and JP-A-53-114717. Showa 57-70
258 and JP-A-59-31827. Further, in order to combine good deep drawability and BH property, Ti and Nb are excessively added in an atomic equivalent amount as compared with the C and N contents to improve the deep drawability, raise the annealing temperature and dissolve the carbide. For example, Japanese Patent Application Laid-Open No. 61-27693 discloses a production method in which a proper amount of solid solution C is left by devising subsequent cooling.
1, disclosed in JP-A-7-300623.

However, the addition of Ti reacts with S, N, and C, and the amount of the reaction varies depending on the slab heating temperature. Therefore, it becomes difficult to adjust the amount of solute C necessary for designing BH properties. Further, in the case of adding Nb and Ti, Nb composite, the amount of solid solution C is adjusted by the atomic ratio of Nb and C.
And C easily react with each other, so that they are susceptible to component variations, particularly the C content, and at the same time, are affected by the cooling rate after annealing, making stable production difficult. In addition, since the recrystallization temperature rises due to the addition of Ti and Nb, high-temperature annealing is required, causing the above-described problem.

[0006] Since Ti and Nb are not added, B is added to suppress nitrogen aging to define B / N, and the cooling rate after recrystallization annealing is restricted to achieve both BH property and aging resistance. This technique is disclosed in JP-A-57-203721. However, simply defining the addition of B and B / N without adding Ti and Nb also causes a small decrease in the recrystallization temperature. Further, rapid aging equipment is required to limit the cooling rate after recrystallization annealing for aging resistance, and stable production is difficult due to the influence of the line speed.

[0007] That is, as the problems of the ultra-low carbon steel sheet that is currently put into practical use, it is necessary to lower the recrystallization temperature, not to raise other manufacturing costs, and not to deteriorate the material such as formability. . In addition, B
Stably producing a non-ageing deep drawn cold-rolled steel sheet having excellent H properties.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an ultra-low carbon steel sheet which can overcome the problems of the prior art.

[0009]

The inventor of the present invention has studied the ultra-low carbon steel which has a lower recrystallization temperature than the conventional one and can be manufactured by low-temperature annealing.
It has been found that by limiting the weight ratio of N to B / N, the rise in recrystallization temperature can be suppressed. Furthermore, it has been found that the balance between the BH property and the aging resistance is improved by performing an appropriate overaging treatment after the recrystallization annealing under an appropriate C content.

The gist of the present invention is as follows. The C content is 0.0020% by weight or less, the atomic ratio B / N between the B content and the N content is 0.6 or more and 1.4 or less, and the precipitate amount ratio between AlN and BN is “AlN / A non-aging deep drawn cold-rolled steel sheet having excellent baking hardenability, wherein "BN" is 0.05 or more and 1.0 or less.

In weight%, C: 0.0020% or less,
Si: 0.5% or less, Mn: 2.0% or less, P: 1.0
%, S: 0.03% or less, O: 0.01% or less, A
l: 0.005% or more and 0.07% or less, B: 0.000
5% or more and 0.005% or less, N: 0.0005% or more and 0.005% or less, the balance consisting of Fe and unavoidable impurities, and the “B / N” is 0.6 or more and 1.4 or less.
N precipitate amount ratio “AlN / BN” is 0.05 or more and 1.0
A non-ageing deep drawn cold-rolled steel sheet having excellent paint bake hardenability, characterized in that:

A C content of 0.0012% by weight or more,
0.0020% by weight or less, the atomic ratio B / N of the B content and the N content is 0.6 or more and 1.4 or less, and AlN and BN
After the recrystallization annealing, the steel sheet after cold rolling having a precipitate amount ratio “AlN / BN” of 0.05 to 1.0 is subjected to an overaging treatment at a temperature of 250 ° C. to 500 ° C. for 30 seconds or more. A method for producing a non-aging deep drawn cold-rolled steel sheet having excellent baking hardenability.

C: 0.0012% or more and 0.0020% or less, Si: 0.50% or less, Mn:
2.0% or less, P: 1.0% or less, S: 0.03% or less, O: 0.01% or less, Al: 0.005% or more.
07% or less, B: 0.0005% to 0.005%, N: 0.0005% to 0.005%, balance F
e and unavoidable impurities, the “B / N” is 0.6 or more and 1.4 or less, and the ratio of the amount of AlN to BN precipitates “AlN
/ BN ”is 0.05 to 1.0, and after recrystallization annealing, an overageing treatment is performed at 250 ° C to 50 ° C.
A method for producing a non-ageing deep drawn cold-rolled steel sheet having excellent paint bake hardenability, which is carried out at 0 ° C. or lower for 30 seconds or more.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
Generally, to lower the recrystallization temperature of a metal, it is sufficient to increase the frequency of recrystallization nucleation and increase the growth rate of recrystallized grains. Therefore, generally, the recrystallization temperature is lowered by reducing impurity elements and reducing solid solution atoms and precipitates. In fact, it is known that recrystallization starts even at 500 ° C. or less in pure iron, and the recrystallization temperature is lowered even when ultra-low carbon steel sheets currently in practical use are not added with Ti or Nb. However, its recrystallization temperature is about 700 ° C., which does not reach 650 ° C. of ordinary low carbon steel. Furthermore, mere addition of Ti and Nb causes deterioration of materials such as moldability. In other words, it became clear that the desired goal could not be achieved with mere purification. This is presumably because the precipitation behavior in practical steel has a delicate effect on the recrystallization temperature and greatly affects the material such as formability. In this developed steel, the atomic ratio of B to N, AlN and B
It has been found that by limiting the weight ratio of N, it is possible to form a precipitate that can achieve both a reduction in the recrystallization temperature and the material of the steel sheet. That is, it has been clarified that the BN precipitation during hot rolling can be promoted by limiting the atomic ratio of B to N to limit the precipitation of fine AlN, thereby suppressing the rise in recrystallization temperature and the deterioration of the material.

The control method of AlN / BN is as follows.
A method of controlling the contents of Al, N, and B and controlling the slab heating temperature in hot rolling can be considered. In controlling the contents of Al, N, and B, the amount of precipitate changes depending on the balance of the three elements. When N and B are constant, the larger the amount of Al, the more Al
The N deposition amount increases, and AlN / BN increases. At the slab heating temperature in hot rolling, the lower the temperature, the greater the amount of BN precipitated and the smaller the AlN / BN. By these combinations, AlN / BN can be controlled.

In the case of ultra-low carbon steel, if no carbide forming element such as Ti or Nb is added, C is considered to be present in the steel in a solid solution state. Although it is not clear yet, it is considered as follows. This solid solution C mainly exists in the crystal grain boundaries, and diffuses into the crystal grains during the coating baking treatment (170 ° C. × 30 minutes), and the yield point rises due to strain age hardening due to dislocations introduced during press working. I do. However, in room temperature aging or aging treatment at 100 ° C. for 60 minutes, which is a problem in aging resistance, solid solution C does not diffuse into crystal grains.
The strain age hardening is small and the aging resistance is secured. Further, the amount of solute C, which significantly affects the rise in the yield point in paint bake hardening, is also determined only by controlling the C content, and a stable BH amount can be secured as compared with the prior art. Therefore, BH property can be imparted by setting an appropriate C content.
Furthermore, by performing overaging treatment after recrystallization annealing, aging resistance and BH balance are improved. The reason for this is not clear, but is considered as follows. Due to the overaging treatment, the slightly dissolved solid C in the grains is agglomerated at the grain boundaries, and the amount of solid solution C diffused into the crystal grains by the coating baking treatment (170 ° C. × 30 minutes) increases. Thereby, the BH property is improved.

The reasons for limiting the components of the present invention are as follows. The reason why the upper limit of the content of C is set to 0.0020% by weight or less is that if the content is more than that, the formability and aging properties of the ultra-low carbon steel sheet are not satisfied, and the recrystallization temperature is increased. is there. Further, aging resistance is not satisfied. The atomic ratio of the B content to the N content "B
/ N "is limited to 0.6 or more and 1.4 or less because B / N below this value causes N aging and particularly causes deterioration of deep drawability. In addition, B /
In N, the recrystallization temperature rises remarkably. Although the contents of B and N are not particularly specified, N is about 0.0005 to 0.005% by weight at the level of a normal ultra-low carbon steel sheet.
Is similar.

The weight ratio of AlN to BN "AlN / BN"
The reason for limiting the thickness to 0.05 or more and 1.0 or less is to lower the recrystallization temperature and improve the formability as shown in FIGS. That is, when this ratio is 1.0 or more,
This is because the effect of increasing the recrystallization temperature by AlN increases, and if the ratio is less than 0.05, the material is not stable and cannot be used as practical steel. Here, the amount ratio of the precipitates of AlN and BN is determined by NasAlN and NasBN obtained from the extraction residue (bromethanol method) of the steel. Although the Al content is not particularly specified,
Al is an essential element as a deoxidizing element for reducing oxygen in molten steel during steelmaking. For that purpose, it needs to be added in an amount of 0.005% by weight or more, and usually up to about 0.07% by weight.

It is needless to say that elements other than those specified above are preferably not added in principle, but Si, Mn, P, and
S, O and the like. Si, Mn, and P are used as solid solution strengthening elements in high-strength steel sheets and the like, and may be added to increase the strength of the steel sheets. However, excessive addition causes an increase in the recrystallization temperature. 0.5% by weight
, Mn is preferably up to about 2.0% by weight, and P is preferably up to 1.0% by weight. S and O have the effect of raising the recrystallization temperature, but the reduction of these elements raises the cost of steelmaking.
In the present invention, S is allowable up to about 0.03% by weight, and O is allowable up to about 0.01% by weight.

By satisfying the above conditions, a non-aging deep drawn cold-rolled steel sheet which can be manufactured by low-temperature annealing and has a low recrystallization temperature can be realized. Further, the following restrictions are required to obtain the BH property. The reason why the lower limit of the C content is set to 0.0012% or more is that a sufficient BH property cannot be obtained with a C content lower than 0.0012%.

The overaging temperature after the recrystallization annealing is 250 ° C.
The reason why the temperature is set to 500 ° C. or less is that C does not diffuse sufficiently at a temperature of 250 ° C. or less. At a temperature of 500 ° C. or higher, aggregation of solid solution C at the grain boundary is inhibited due to excessive diffusion. The reason for setting the overaging treatment time to 30 seconds or longer is that if the time is shorter than this, the coagulation of the solid solution C at the grain boundaries becomes insufficient.

By satisfying the above conditions, a steel sheet which can be manufactured by low-temperature annealing and has excellent BH properties and aging resistance can be realized.

[0023]

EXAMPLE A continuously cast slab having the composition shown in Table 1 was heated at 1050 to 1200 ° C., finish rolled at 910 ° C., cooled, and then rolled at about 650 ° C. to obtain a 4 mm thick hot rolled sheet. %, And then recrystallization annealing was performed. First, in order to measure the recrystallization temperature, a cold-rolled steel sheet with a thermocouple is immersed in a salt bath maintained at a predetermined temperature, and taken out of the salt bath 60 seconds after the steel sheet temperature reaches the predetermined temperature. Mirrored. At the time of microscopy, the recrystallization rate was evaluated by the point calculation method, and the temperature at which recrystallization was completed was determined. A recrystallization temperature of 700 ° C. or less was considered good. Further, an extraction residue was obtained from the steel sheet in a state where the recrystallization was completed by a bromethanol method, and Nas Al was obtained.
N and N as BN are determined, and from the ratio, “AlN / B
The "N" value was evaluated. Next, recrystallization annealing for evaluating the Rankford value, BH property, and aging resistance was performed by continuous annealing. At this time, some samples were overaged.
Further, for some samples, hot dip galvanizing was performed after the recrystallization annealing was completed. 0 for the material after continuous annealing.
After skin pass rolling of 8%, a Rankford value, BH property, and aging resistance were evaluated using a JIS No. 5 tensile test piece. The BH property was 170 ° C. after applying 2% tensile strain.
Evaluation was made based on the amount of increase in deformation stress after aging treatment for × 20 minutes.
The aging resistance was evaluated by the elongation at the yield point in the tensile test in the rolling direction after aging treatment at 100 ° C. for 60 minutes. Table 2 shows the test results of the samples without overaging treatment. Table 3 shows the overaging conditions and test results of the overaged samples. Here, it was determined that the BH amount was good when the BH amount was 30 MPa or more and the increase in the BH amount due to overaging treatment was 7 MPa or more.

[0024]

[Table 1]

In Table 2, Experiment Nos. 1 to 7 are experiments in which the C content was changed. Experiment Nos. 1 to 6 satisfied the condition of claim 1 of the present invention, and a steel plate having a low recrystallization temperature and good workability was produced. In Experiment 3, galvannealing was performed after recrystallization annealing. In Experiment 5, electrogalvanizing was performed after skin pass rolling was completed. Thus, regardless of the surface treatment, if the condition of claim 1 of the present invention is satisfied, a steel plate having a low recrystallization temperature and good workability can be manufactured. In Experiment No. 7, since the C content was more than the limit, the aging resistance was deteriorated. Experiment Nos. 8 and 9 are steel sheets which were subjected to solid solution strengthening with Si, Mn, and P, satisfying the condition of claim 1 of the present invention, and were able to produce steel sheets having a low recrystallization temperature and good workability. . Experiment number 1
Nos. 0 to 12 are steel sheets containing Ti and Nb, which are conventional steels, but their recrystallization temperatures increased. Experiment number 13 is B / N
Was higher than the limit, and the recrystallization temperature increased. In Experiment No. 14, since the B / N was less than the limit, the aging resistance was deteriorated. In Experiment No. 15, the recrystallization temperature increased because AlN / BN was higher than the limit. It is considered that the reason why the ratio of AlN / BN exceeded the limit was that the Al content was higher than the hot rolling conditions in this case. Experiment number 16 is Al
Since N / BN was less than the limit, the material deteriorated and the r value became low. It is considered that the reason why the ratio of AlN / BN was lower than the limit was that the Al content was smaller than the hot rolling conditions in this case.

[0026]

[Table 2]

In Table 3, Experiment Nos. 16 to 20 are experiments in which the effect of overaging conditions was examined using steel type E. Since Experiment Nos. 17 and 18 satisfied the condition of Claim 2 of the present invention, a sufficient BH amount and an improvement in BH amount were observed. In Experiment Nos. 16 and 19, the overaging temperature was higher than the limit,
Since it was below the limit, a sufficient improvement in the BH amount could not be obtained. In Experiment No. 20, since the overaging time was less than the limit, a sufficient improvement in the BH amount was not obtained. Experiment number 21
To 23 are experiments in which the effect of the C content was observed. In Experiment No. 21, since the C content was below the limit, neither a sufficient BH amount nor a BH amount improvement was observed. Experiment No. 22 satisfies the condition of claim 2 of the present invention, and thus a sufficient BH amount and an improvement in the BH amount were observed. In Experiment No. 23, since the C content was more than the limit, the aging resistance was deteriorated.

[0028]

[Table 3]

[0029]

According to the present invention, a non-ageing deep-drawn cold-rolled steel sheet used for various purposes such as for automobiles, home appliances, containers, etc. can be stably supplied at low cost by low-temperature annealing. This is an invention of high industrial value, in which a cold-rolled steel sheet such as an outer plate that requires BH properties and aging resistance can be stably supplied at low cost by low-temperature annealing.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an AlN / BN ratio and an r value.

FIG. 2 is a diagram showing a relationship between an AlN / BN ratio and a recrystallization temperature.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masayoshi Suehiro 1-1 Niwahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Nippon Steel Corporation Yawata Works (72) Inventor Masaharu Kameda Tobata-ku, Tobata-ku, Kitakyushu-shi, Fukuoka Town 1-1 Inside Nippon Steel Corporation Yawata Works

Claims (4)

[Claims] 1. The method according to claim 1, wherein the C content is 0.0020% by weight or less,
The atomic ratio “B / N” between the B content and the N content is 0.6 or more and 1.4 or less, and the ratio of the amount of AlN to BN precipitates “AlN
/ BN "is 0.05 or more and 1.0 or less, and is a non-aging deep drawn cold-rolled steel sheet having excellent baking hardenability. 2. In% by weight, C: 0.0020% or less, S
i: 0.5% or less, Mn: 2.0% or less, P: 1.0%
Hereinafter, S: 0.03% or less, O: 0.01% or less, A
l: 0.005% or more and 0.07% or less, B: 0.000
5% or more and 0.005% or less, N: 0.0005% or more and 0.005% or less, the balance consisting of Fe and unavoidable impurities, and the “B / N” is 0.6 or more and 1.4 or less.
N precipitate amount ratio “AlN / BN” is 0.05 or more and 1.0
A non-ageing deep drawn cold-rolled steel sheet having excellent paint bake hardenability, characterized in that: 3. C content is 0.0012% by weight or more,
0.0020% by weight or less, the atomic ratio B / N of the B content and the N content is 0.6 or more and 1.4 or less, and AlN and BN
After the recrystallization annealing, the steel sheet after cold rolling having a precipitate amount ratio “AlN / BN” of 0.05 to 1.0 is subjected to an overaging treatment at a temperature of 250 ° C. to 500 ° C. for 30 seconds or more. A method for producing a non-aging deep drawn cold-rolled steel sheet having excellent baking hardenability. 4. C: 0.0012% to 0.1% by weight.
0020% or less, Si: 0.50% or less, Mn: 2.0
%, P: 1.0% or less, S: 0.03% or less, O:
0.01% or less, Al: 0.005% to 0.07%, B: 0.0005% to 0.005%, N:
"B / N" is comprised between 0.0005% and 0.005%, the balance being Fe and unavoidable impurities, and "B / N" is between 0.6 and 1.4, and the precipitate amount ratio of AlN to BN "AlN / BN"
Is from 0.05 to 1.0 after the cold-rolled steel sheet,
A method for producing a non-ageing deep-drawn cold-rolled steel sheet having excellent paint bake hardenability, wherein an overaging treatment is performed at 250 ° C or more and 500 ° C or less for 30 seconds or more after recrystallization annealing.