JPH0693376A - Cold rolled sheet of ferrite single phase for deep drawing having nonaging characteristic at ordinary temperature and its production - Google Patents

Abstract

PURPOSE:To provide superior secondary working brittleness resistance and hardenability in coating/backing by controlling respective contents of C, P, and B to values in specific ranges in a steel free from Ti and Nb. CONSTITUTION:The steel has a composition consisting of, by weight, 0.0001-O.0015% C, <=1.2% Si, 0.03-3.0% Mn, 0.01-0.15% p, 0.0010-0.020% S, 0.005-0.1% Al, 0.001-0.0080% N, 0.0001-0.0030% B, and the balance Fe with inevitable impurities. A slab of the steel with this chemical composition is finish-hot-rolled at >=(Ar3-100 deg.C) and coiled at 600-700 deg.C. Subsequently, the resulting plate us cold-rolled at >=60% rolling rate and continuously annealed at 600-900 deg.C, by which a cold rolled sheet of ferrite single phase for deep drawing having nonaging characteristic at ordinary temp. can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferrite single-phase cold-rolled steel sheet for non-aging deep drawing at room temperature and a method for producing the same. The cold-rolled steel sheet according to the present invention is used by press forming automobiles, household electric appliances, buildings and the like. Further, it includes both a cold-rolled steel sheet in a narrow sense that is not surface-treated and a cold-rolled steel sheet that has been subjected to surface treatment such as Zn plating or alloyed Zn plating for rust prevention. Since the steel sheet according to the present invention is a steel sheet having both strength and workability, it is possible to reduce the plate thickness in use, that is, to reduce the weight, in use. Therefore, it is expected to contribute to global environment conservation.

[0002]

2. Description of the Related Art Recent advances in vacuum degassing of molten steel have facilitated the melting of ultra-low carbon steel, and nowadays, the demand for ultra-low carbon steel sheets having good workability is increasing.
It is well known that such an ultra-low carbon steel sheet generally contains at least one of Ti and Nb. That is, Ti and Nb have a strong attractive interaction with the interstitial solid solution elements (C, N) in steel, and easily form carbonitrides. Therefore, steel without interstitial solid solution elements (IF steel: Interstitial Free)
Steel) is obtained. Since the IF steel does not contain an interstitial solid solution element that causes strain aging or deterioration of workability, it has a characteristic of being non-aging and having very good workability.
Furthermore, the addition of Ti or Nb also has an important role of improving the deep drawability of the cold-rolled annealed sheet by reducing the grain size of the hot-rolled sheet of ultra-low carbon steel that tends to coarsen. However, Ti and Nb
The ultra-low carbon steel added with has the following problems. First, the manufacturing cost is high. That is, it is necessary to add expensive Ti and Nb in addition to the vacuum processing cost for extremely low carbonization. Second, solid solution C and N on the product plate
Does not remain, so that secondary processing embrittlement occurs and paint baking hardening disappears. Thirdly, Ti and Nb are strong oxide forming elements, and these oxides deteriorate the surface quality.

[0003] A lot of research and development has been conducted so far for the purpose of solving such problems of IF steel. For example,
JP-A-60-197846 and JP-A-63-7
Japanese Patent No. 2830 discloses an ultra-low carbon steel sheet to which Ti or Nb is not added and a method for producing the same. Basically, when continuously annealing a steel having a C content of 0.0010 to 0.0080%, high temperature annealing is performed. The above problem is solved by temporarily transforming a part of α into γ by using, and producing a low temperature transformation product from γ by controlling the cooling rate, and forming a mixed structure of this and α. . However, for ultra low carbon steel (α + γ)
The two-phase region is extremely narrow, it is difficult to control the temperature with high accuracy, and various problems associated with high temperature annealing, such as poor hot stripability, poor plate shape, and high energy consumption, are problems. Occur. Therefore, the steel sheet of the present invention is assumed to have an α single phase structure. In addition, JP-A-59-
80727, JP-A-60-103129,
In JP-A-1-184251 and the like, the amount of C is 0.0015 without adding expensive elements such as Ti and Nb.
Disclosed is a cold-rolled steel sheet including a region of not more than%, and a manufacturing method thereof. However, in these cases, B, which is one of the features of the present invention, is not added. Total C amount is 0.00
If it is 15% or less, even if Ti or Nb is not added, C existing in the crystal grain boundaries is extremely reduced, and secondary work embrittlement occurs. Furthermore, JP-A-58-14133
In Japanese Patent No. 5, gazette contains a region where the amount of C is 0.0015% or less, and 0.0005 to 0.0020% of B is added. However, when the amount of C is in the range of 0.0015% or less, the crystal grain size of the hot-rolled sheet is generally large, and the r value of the cold-rolled annealed sheet cannot be secured. Therefore, some measure is required for the additive element or the hot rolling method.

[0004]

As is apparent from the above, the present inventors and of course, in the field of the present invention, using ultra-low carbon steel which does not use expensive additive elements such as Ti and Nb. It has long been required to establish a cold-rolled steel sheet for room-temperature non-aged drawing which has secondary work embrittlement resistance and also has a paint bake hardening performance, and a manufacturing method thereof.

[0005]

[Means for Solving the Problems] As one means for achieving room-temperature non-aging characteristics without using expensive carbonitride-forming elements such as Ti and Nb, a pole whose total C content is controlled below a certain amount is used. A method using low carbon Al killed has been found. That is, when the steel of the present invention is used, N can be fixed as AlN by adding Al in the product plate, so that C causes strain aging. As a result of diligent research and development by the present inventors, when the total C amount is 15 ppm or less, even if the temper rolling ratio is 0.5% and is lower than usual, a stable non-aging is performed at room temperature. There was found. Further, as described above, the present invention is premised on the steel sheet having the α single phase structure.

Next, the secondary working embrittlement resistance will be described.
The problem of secondary processing embrittlement is that the total C content is 15 pp as described above.
It has been found that when it is less than m, it tends to occur. It is considered that this is because the amount of C that strengthens the grain boundaries was significantly reduced. Further, as will be described later, when P is added to improve the deep drawability or increase the strength, this problem becomes more severe. As a means to solve this problem,
It has been found for the first time that B addition is effective even in an extremely low carbon steel to which elements such as Ti and Nb are not added as in the present invention.

Thirdly, a method for improving the deep drawability of an ultra low carbon steel sheet to which elements such as Ti and Nb are not added will be described. Generally, in steels without addition of Ti or Nb, the total C
When the amount is reduced, the crystal grain size of the hot-rolled sheet becomes large, particularly when the total C amount is in the range of 15 ppm or less, it becomes remarkably large, and sometimes it becomes extremely coarse columnar crystals extending in the sheet thickness direction. However, the plate plane {111} oriented grains, which are preferable for deep drawability, preferentially nucleate from the initial grain boundaries, so the r value is rather lowered even if the carbon is extremely low. Therefore, Ti and Nb
As a result of investigating a method of refining the crystal grain size of the hot-rolled sheet without adding an expensive element, 1) P addition is effective, and 0.01% or more is preferable,
2) When coexisting with B, this effect becomes more remarkable 3)
More preferably, within 50 seconds after completion of hot rolling, 50
It was found that the particles became finer when cooled at a cooling rate of ° C / sec or more. Regarding the above 1), the reason is not always clear, but it is presumed that it was caused by adding P, a) the γ grains became finer, and b) the grain growth of transformed α was suppressed. . On the other hand, it is considered that the addition of B suppresses the transformation speed, and therefore the α-grain size after transformation becomes small. Further, it is considered that the quenching after the hot rolling finish was effective for the grain refinement by suppressing the grain growth and increasing the γ / α ratio.

The present invention is constructed on the basis of such an idea and new knowledge, and the gist thereof is as follows.
It is as follows. (1) C: 0.0001 to 0.0015% by weight,
Si: 1.2% or less, Mn: 0.03 to 3.0%, P:
0.01-0.15%, S: 0.0010-0.020
%, Al: 0.005-0.1%, N: 0.0001-
0.0080%, B: 0.0001 to 0.0030%, excellent in secondary processing embrittlement resistance consisting of balance Fe and unavoidable impurities, and has a paint bake hardening property. Phase cold rolled steel sheet. (2) A slab composed of the chemical components described in 1 above (Ar ₃
Finish the hot rolling at a temperature of -100) ℃ or more,
A ferrite single-phase cold-rolled steel sheet for non-aging deep drawing at room temperature, which is wound at a temperature of 00 to 750 ° C., cold-rolled at a rolling ratio of 60% or more, and continuously annealed at 600 to 900 ° C. Manufacturing method.

[0009]

The reason why the steel composition and manufacturing conditions are limited as described above in the present invention will be further described. 1) C: C is an extremely important element that determines the material properties of products. When the amount of C exceeds 0.0015%, it is no longer normal temperature non-aged, so the upper limit is made 0.0015%. On the other hand, if the C content is less than 0.0001%, secondary working embrittlement occurs. In addition, it is an extremely difficult area to reach in terms of steelmaking technology, and the cost increases significantly. Therefore, the lower limit is made 0.0001%.

2) Si: Si is an element that increases the strength at a low cost, but if it exceeds 1.2%, problems such as deterioration of chemical conversion treatment property and plating property occur, so its upper limit is 1.2. %. 3) Mn: Mn is an element effective for increasing the strength like Si. Further, in the steel of the present invention to which Ti or the like is not added, Mn fixes S, so Mn has a role of preventing cracks during hot rolling. It has been conventionally said that lowering Mn is preferable for improving the r value, but if the Mn content is less than 0.03%, cracking occurs during hot rolling. Therefore, the lower limit of the amount of Mn is set to 0.03%. On the other hand, a new finding was obtained that Mn coexists with P to reduce the grain size of the hot-rolled sheet of ultra-low carbon steel. This is because both elements act thermodynamically in the direction of canceling the Ar ₃ temperature, and both elements delay the transformation from γ to α kinetically. Therefore, when the Mn content is increased remarkably, the r value generally deteriorates sharply, but the P content is 0.01% as in the present invention.
The useful knowledge that the above ultra-low carbon steel does not deteriorate so much even when added up to 3.0% was obtained. For the above reasons,
The upper limit of the amount of Mn is 3.0%.

4) P: P is also known as an element for increasing the strength like Si and Mn, and the addition amount thereof changes depending on the target strength level. In addition, Ti and Nb
The crystal grain size of the ultra-low carbon steel hot-rolled sheet without addition of Cr generally becomes coarse, but the addition of 0.01% or more of P causes
We have obtained a new finding that the particles are noticeably finer. Therefore, P
The lower limit of the amount is 0.01%. However, when the addition amount is 0.
If it exceeds 15%, deterioration of cold rolling property, secondary work embrittlement and the like occur, so the upper limit of the amount of P is made 0.15%.
Also, as described in 3) above, the grain refining effect of P is M
It becomes more remarkable when coexisting with n.

5) S: The lower the S content, the better,
If it is less than 0.0010%, the manufacturing cost increases, so this is made the lower limit. On the other hand, if it exceeds 0.020%, a large amount of MnS precipitates and the workability deteriorates, so this is made the upper limit. 6) Al: Al is used for deoxidation adjustment, but 0.005%
If it is less than 0.1, it becomes difficult to perform stable deoxidation. on the other hand,
If it exceeds 0.1%, the cost increases. Therefore, these values are set as the lower limit and the upper limit.

7) N: N is preferably low. But,
If it is less than 0.0001%, a significant cost increase will occur, so this is made the lower limit. On the other hand, if it exceeds 0.0080%, it becomes difficult to fix N with Al,
Workability deteriorates because the solid solution N that causes the strain aging remains or the fraction of AlN increases. Therefore, 0.
Let 0080% be the upper limit of the N amount.

8) B: B segregates at the grain boundaries and is effective in preventing secondary work embrittlement. The effect is 0.0001-
Addition of 0.0030% is sufficient. If it is less than 0.0001%, the effect is insufficient, and if it exceeds 0.0030%, the addition cost increases and slab cracking occurs. Next, the reasons for limiting the manufacturing conditions will be described.

9) Finishing temperature of hot rolling: Workability of product sheet
In order to secure (r value), (Ar ₃-100) ℃ or more
Finish at the temperature of. Also, within 50 seconds after finishing,
Crystals of hot-rolled sheet when rapidly cooled at a cooling rate of over 1 / sec
Such conditions are preferable because the particle size becomes finer. 10) Winding temperature: When it exceeds 750 ° C, the pickling property deteriorates.
This is because the material becomes uneven in the longitudinal direction of the coil.
Is the upper limit. On the other hand, when the temperature is lower than 600 ° C, hot rolling is performed.
Since the precipitation of AlN on the plate becomes insufficient, processing of the product plate
Sex deteriorates. Therefore, this is the lower limit.

11) Cold rolling: Normal conditions may be used, and the rolling reduction is 60% or more for the purpose of ensuring the r value of the product sheet. 12) Annealing: Continuous annealing is performed at an annealing temperature of 600 to 900 ° C. If the annealing temperature is lower than 600 ° C., recrystallization is insufficient and the workability of the product sheet becomes a problem. Although the workability improves as the annealing temperature rises, if it exceeds 900 ° C, the temperature will be too high and the plate fracture and the flatness of the plate will deteriorate.

Thus, the present invention is constructed based on new ideas and new findings, and according to the present invention, Ti and N are
Even if an expensive element such as b is not added, a cold-rolled steel sheet for deep drawing which is non-aging at room temperature, has a paint bake hardenability, and is excellent in secondary work embrittlement resistance can be obtained.

[0018]

【Example】

Example 1 A steel having the composition shown in Table 1 was vacuum-melted in a laboratory.
That is, steel A (A-1 to A-5) and steel B (B-1 to B)
-5) group has a C content of 0.0003% to 0.0
Five levels have changed to 030%. Here, the amount of P is 0.015% for steel A and 0.050% for steel B.
On the other hand, Steel C (C-1 to C-6) and Steel D (D-1 to D-
In the group of 6), the amount of P is 0.0002% to 0.04.
6 levels are changing up to%. Here, steel C has a C content of 0.
0005% and Steel D has a C content of 0.0012%. An ingot having such a chemical composition is heated at a slab temperature of 1150 ° C., a finishing temperature of 910 ° C., and a winding temperature of 710.
It was hot-rolled at ℃ to obtain a steel plate having a thickness of 4.0 mm. 8 after pickling
Cold rolling of 0% reduction rate is performed to make a 0.8 mm cold rolled sheet, and then heating rate is 15 ° C./sec, soaking is 780 ° C. × 5.
Continuous annealing was performed at 0 sec and a cooling rate of 20 ° C./sec.
Further, it was temper-rolled with a rolling reduction of 0.8% and subjected to a tensile test. The tensile test method was according to the method described in JIS2241. The paint bake hardenability (BH property) is the amount of increase in the yield point when the tensile test is performed again after 170% -20 min baking equivalent treatment after 2% tensile prestrain.

As is clear from FIG. 1, when the total C content is 0.0015% or less without adding Ti, Nb, or the like,
Yield point elongation (YP-El) after 100 ° C-1 hr is 0.
It will be less than 2% and achieve the target of non-aging at room temperature. Further, when the total amount of C is 0.0001% or more, it becomes possible to impart the BH property which is very difficult with the ultra-low carbon steel to which Ti or Nb is added. On the other hand, as is clear from FIG. 2, when the P addition amount is 0.01% or more, the low r value, especially r ₄₅ , which is a drawback of the ultra-low carbon steel containing no Ti or Nb, is remarkably improved, and the deep drawing is performed. It is a sufficient level as a steel plate for use.

Example 2 Based on the findings of Example 1, it has the chemical composition shown in Table 2.
Steel is melted and cast on a full scale, followed by hot rolling (heating
Temperature: 1200 ° C, finishing temperature: 930 ° C, winding temperature: 7
10 ° C), cold rolling (reduction: 80%), continuous annealing (7
80 ℃ -40sec retention and 400 ℃ -2min overtime
It was subjected to tempering treatment) and temper rolling (0.8%). Pull
The tension test followed the same method as in Example 1. Also secondary
As for workability, the annealed plate was punched into a disk and the draw ratio was 1.6.
The material that has been changed to various temperatures,
Place it on the tool and drop a 300kg weight from a height of 1m.
Ductility-brittle transition temperature when shock is applied and fractured as
The value of -20 ° C or lower was evaluated as good. Table 3
As is clear from the above, according to the present invention, Ti, Nb, etc.
Strength level using steel without the addition of expensive elements
Is 30 kgf / mm ²To 45 kgf / mm²Up to
Cold rolled steel sheet for non-aging deep drawing at room temperature is obtained, and BH performance is also
It is possible to combine both. Also, for the addition of a small amount of B
It can be seen that the secondary work embrittlement resistance is significantly improved.
It Here, in the steels 3-1, 3-2, P and Mn are simultaneously added.
Although it has a higher strength, it has a high Mn.
Good r, r₄₅Becomes This is a simultaneous P and Mn
It is thought that the addition is effective for making the hot-rolled sheet finer.
To be

Example 3 Using the steels 1-1 and 2-1 in Table 2, the cooling conditions after completion of hot rolling were examined using actual equipment. Table 4 shows the relationship between hot rolling conditions and r and r ₄₅ of the product sheet. Here, as hot rolling conditions, the cooling conditions after finishing, particularly the time until the start of quenching and the cooling rate were examined. The cold rolling has a reduction rate of 80% and a plate thickness of 0.8 mm. Continuous annealing at 780 ° C.-40 sec,
And subjected to temper rolling with a rolling reduction of 0.8%. As is clear from Table 4, r and r ₄₅ as a deep-drawing steel sheet are satisfied even under normal conditions, but preferably r, particularly r ₄₅ is remarkably improved by rapid cooling as soon as possible after completion of hot rolling. It is considered that this is because the crystal grain size of the hot rolled plate is refined by rapid cooling immediately after hot rolling.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

[0026]

As described above in detail, according to the present invention, T
A cold-rolled steel sheet that is not aged at room temperature and has excellent deep drawability can be obtained without adding an expensive element such as i or Nb, and secondary work embrittlement resistance and paint bake hardenability can also be imparted. Further, the present invention can be applied to a surface-treated steel sheet subjected to electroplating, hot dip plating, etc., and its production. As described above, the present invention not only enables the production of a steel sheet having excellent performance stably and inexpensively as compared with the prior art, but also secures earth resources of expensive elements, or according to the present invention. The use of high-strength steel sheets is considered to contribute to global environmental conservation, and the effect is remarkable.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the BH amount and YP-El (after aging at 100 ° C. for 1 hr) and the C amount.

FIG. 2 is a diagram showing the relationship between r and r ₄₅ and the amount of P.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Akisue 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Co., Ltd. Corporate Technology Development Division (72) Inventor Kunio Nishimura 1-1 Hibahata-cho, Tobata-ku, Kitakyushu, Fukuoka No. New Nippon Steel Co., Ltd., Yawata Works

Claims (2)

[Claims] 1. C: 0.0001 to 0.00 in% by weight.
15%, Si: 1.2% or less, Mn: 0.03 to 3.0
%, P: 0.01 to 0.15%, S: 0.0010
0.020%, Al: 0.005-0.1%, N: 0.
0001 to 0.0080%, B: 0.0001 to 0.0
Ferrite single-phase cold-rolled steel sheet for normal temperature non-aged deep drawing which contains 030% and is excellent in secondary work embrittlement resistance consisting of balance Fe and unavoidable impurities and has paint bake hardening characteristics. 2. A slab made of the chemical composition according to claim 1 is hot-rolled at a temperature of (Ar ₃ -100) ° C. or higher and wound at a temperature of 600 ° C. to 750 ° C.
A method for producing a ferrite single-phase cold-rolled steel sheet for normal temperature non-aged deep drawing, which comprises performing cold rolling at the above-mentioned rolling ratios and continuous annealing at 600 to 900 ° C.